diff --git a/mesa/batchrunner.py b/mesa/batchrunner.py
index 0556a38..b4290f2 100644
--- a/mesa/batchrunner.py
+++ b/mesa/batchrunner.py
@@ -128,10 +128,14 @@ def _make_model_kwargs(
     """
     parameter_list = []
     for param, values in parameters.items():
-        try:
-            all_values = [(param, value) for value in values]
-        except TypeError:
+        if isinstance(values, str):
+            # The values is a single string, so we shouldn't iterate over it.
             all_values = [(param, values)]
+        else:
+            try:
+                all_values = [(param, value) for value in values]
+            except TypeError:
+                all_values = [(param, values)]
         parameter_list.append(all_values)
     all_kwargs = itertools.product(*parameter_list)
     kwargs_list = [dict(kwargs) for kwargs in all_kwargs]
diff --git a/mesa/visualization/ModularVisualization.py b/mesa/visualization/ModularVisualization.py
index e9d2548..3c302ee 100644
--- a/mesa/visualization/ModularVisualization.py
+++ b/mesa/visualization/ModularVisualization.py
@@ -116,7 +116,7 @@ from mesa.visualization.UserParam import UserSettableParameter
 if platform.system() == "Windows" and platform.python_version_tuple() >= ("3", "7"):
     asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
 
-D3_JS_FILE = "external/d3-4.13.0.min.js"
+D3_JS_FILE = "external/d3-7.4.3.min.js"
 CHART_JS_FILE = "external/chart-3.6.1.min.js"
 
 
@@ -125,10 +125,10 @@ class VisualizationElement:
     Defines an element of the visualization.
 
     Attributes:
-        package_includes: A list of external JavaScript files to include that
-                          are part of the Mesa packages.
-        local_includes: A list of JavaScript files that are local to the
-                        directory that the server is being run in.
+        package_includes: A list of external JavaScript and CSS files to
+                          include that are part of the Mesa packages.
+        local_includes: A list of JavaScript and CSS files that are local to
+                        the directory that the server is being run in.
         js_code: A JavaScript code string to instantiate the element.
 
     Methods:
@@ -174,8 +174,10 @@ class PageHandler(tornado.web.RequestHandler):
             port=self.application.port,
             model_name=self.application.model_name,
             description=self.application.description,
-            package_includes=self.application.package_includes,
-            local_includes=self.application.local_includes,
+            package_js_includes=self.application.package_js_includes,
+            package_css_includes=self.application.package_css_includes,
+            local_js_includes=self.application.local_js_includes,
+            local_css_includes=self.application.local_css_includes,
             scripts=self.application.js_code,
         )
 
@@ -266,14 +268,22 @@ class ModularServer(tornado.web.Application):
         """Create a new visualization server with the given elements."""
         # Prep visualization elements:
         self.visualization_elements = visualization_elements
-        self.package_includes = set()
-        self.local_includes = set()
+        self.package_js_includes = set()
+        self.package_css_includes = set()
+        self.local_js_includes = set()
+        self.local_css_includes = set()
         self.js_code = []
         for element in self.visualization_elements:
             for include_file in element.package_includes:
-                self.package_includes.add(include_file)
+                if self._is_stylesheet(include_file):
+                    self.package_css_includes.add(include_file)
+                else:
+                    self.package_js_includes.add(include_file)
             for include_file in element.local_includes:
-                self.local_includes.add(include_file)
+                if self._is_stylesheet(include_file):
+                    self.local_css_includes.add(include_file)
+                else:
+                    self.local_js_includes.add(include_file)
             self.js_code.append(element.js_code)
 
         # Initializing the model
@@ -338,3 +348,7 @@ class ModularServer(tornado.web.Application):
             webbrowser.open(url)
         tornado.autoreload.start()
         tornado.ioloop.IOLoop.current().start()
+
+    @staticmethod
+    def _is_stylesheet(filename):
+        return filename.lower().endswith(".css")
diff --git a/mesa/visualization/templates/css/visualization.css b/mesa/visualization/templates/css/visualization.css
index 38d8850..9ae0af8 100644
--- a/mesa/visualization/templates/css/visualization.css
+++ b/mesa/visualization/templates/css/visualization.css
@@ -2,7 +2,8 @@
     margin-bottom: 15px;
 }
 
-div.tooltip {
+/* This is specific to the Network visualization */
+div.d3tooltip {
     position: absolute;
     text-align: center;
     padding: 1px;
diff --git a/mesa/visualization/templates/js/NetworkModule_d3.js b/mesa/visualization/templates/js/NetworkModule_d3.js
index 7d3839a..110322c 100644
--- a/mesa/visualization/templates/js/NetworkModule_d3.js
+++ b/mesa/visualization/templates/js/NetworkModule_d3.js
@@ -22,7 +22,7 @@ const NetworkModule = function (svg_width, svg_height) {
   const tooltip = d3
     .select("body")
     .append("div")
-    .attr("class", "tooltip")
+    .attr("class", "d3tooltip")
     .style("opacity", 0);
 
   svg.call(
@@ -38,7 +38,7 @@ const NetworkModule = function (svg_width, svg_height) {
   this.render = function (data) {
     const graph = JSON.parse(JSON.stringify(data));
 
-    simulation = d3
+    const simulation = d3
       .forceSimulation()
       .nodes(graph.nodes)
       .force("charge", d3.forceManyBody().strength(-80).distanceMin(2))
@@ -89,12 +89,12 @@ const NetworkModule = function (svg_width, svg_height) {
       .data(graph.nodes)
       .enter()
       .append("circle")
-      .on("mouseover", function (d) {
+      .on("mouseover", function (event, d) {
         tooltip.transition().duration(200).style("opacity", 0.9);
         tooltip
           .html(d.tooltip)
-          .style("left", d3.event.pageX + "px")
-          .style("top", d3.event.pageY + "px");
+          .style("left", event.pageX + "px")
+          .style("top", event.pageY + "px");
       })
       .on("mouseout", function () {
         tooltip.transition().duration(500).style("opacity", 0);
diff --git a/mesa/visualization/templates/js/runcontrol.js b/mesa/visualization/templates/js/runcontrol.js
index e7768aa..89e58ba 100644
--- a/mesa/visualization/templates/js/runcontrol.js
+++ b/mesa/visualization/templates/js/runcontrol.js
@@ -190,10 +190,10 @@ const initGUI = function (model_params) {
     const domID = param + "_id";
     sidebar.append(
       [
-        "<div class='input-group input-group-lg'>",
+        "<div>",
         "<p><label for='" +
           domID +
-          "' class='label label-primary'>" +
+          "' class='badge badge-primary'>" +
           obj.name +
           "</label></p>",
         "<input class='model-parameter' id='" + domID + "' type='checkbox'/>",
@@ -213,10 +213,10 @@ const initGUI = function (model_params) {
     const domID = param + "_id";
     sidebar.append(
       [
-        "<div class='input-group input-group-lg'>",
+        "<div>",
         "<p><label for='" +
           domID +
-          "' class='label label-primary'>" +
+          "' class='badge badge-primary'>" +
           obj.name +
           "</label></p>",
         "<input class='model-parameter' id='" + domID + "' type='number'/>",
@@ -235,11 +235,11 @@ const initGUI = function (model_params) {
     const tooltipID = domID + "_tooltip";
     sidebar.append(
       [
-        "<div class='input-group input-group-lg'>",
+        "<div>",
         "<p>",
         "<a id='" +
           tooltipID +
-          "' data-toggle='tooltip' data-placement='top' class='label label-primary'>",
+          "' data-toggle='tooltip' data-placement='top' class='badge badge-primary'>",
         obj.name,
         "</a>",
         "</p>",
@@ -278,7 +278,7 @@ const initGUI = function (model_params) {
     const template = [
       "<p><label for='" +
         domID +
-        "' class='label label-primary'>" +
+        "' class='badge badge-primary'>" +
         obj.name +
         "</label></p>",
       "<div class='dropdown'>",
diff --git a/mesa/visualization/templates/modular_template.html b/mesa/visualization/templates/modular_template.html
index c185e71..5b75089 100644
--- a/mesa/visualization/templates/modular_template.html
+++ b/mesa/visualization/templates/modular_template.html
@@ -1,12 +1,19 @@
 <!DOCTYPE html>
 <head>
 	<title>{{ model_name }} (Mesa visualization)</title>
-    <link href="/static/external/bootstrap-3.3.7-dist/css/bootstrap.min.css" type="text/css" rel="stylesheet" />
-    <link href="/static/external/bootstrap-3.3.7-dist/css/bootstrap-theme.min.css" type="text/css" rel="stylesheet" />
+    <link href="/static/external/bootstrap-4.6.1-dist/css/bootstrap.min.css" type="text/css" rel="stylesheet" />
     <link href="/static/external/bootstrap-switch-3.3.4/dist/css/bootstrap3/bootstrap-switch.min.css" type="text/css" rel="stylesheet" />
-    <link href="/static/external/bootstrap-slider-9.8.0/dist/css/bootstrap-slider.min.css" type="text/css" rel="stylesheet" />
+    <link href="/static/external/bootstrap-slider-11.0.2/dist/css/bootstrap-slider.min.css" type="text/css" rel="stylesheet" />
     <link href="/static/css/visualization.css" type="text/css" rel="stylesheet" />
 
+    <!-- CSS includes go here -->
+    {% for file_name in package_css_includes %}
+        <link href="/static/css/{{ file_name }}" type="text/css" rel="stylesheet" />
+    {% end %}
+    {% for file_name in local_css_includes %}
+        <link href="/local/{{ file_name }}" type="text/css" rel="stylesheet" />
+    {% end %}
+
 	<!-- This is the Tornado template for the Modular Visualization. The Javascript code opens a WebSocket connection to
 	the server (the port is set via the template). On every step, it receives inputs, one per module, and sends
 	them to the associated function to render. -->
@@ -14,42 +21,43 @@
 <body>
 
     <!-- Navbar -->
-    <nav class="navbar navbar-inverse navbar-static-top">
+    <nav class="navbar navbar-dark bg-dark navbar-static-top navbar-expand-md mb-3">
         <div class="container">
-            <div class="navbar-header">
-                <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
-                    <span class="sr-only">Toggle navigation</span>
-                    <span class="icon-bar"></span>
-                    <span class="icon-bar"></span>
-                    <span class="icon-bar"></span>
-                </button>
+            <button type="button" class="navbar-toggler collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
+                <span class="sr-only">Toggle navigation</span>
+                &#x2630;
+            </button>
             <a class="navbar-brand" href="#">{{ model_name }}</a>
-            </div>
+
             <div id="navbar" class="navbar-collapse collapse">
                 <ul class="nav navbar-nav">
-                    <li>
-                        <a href="#" data-toggle="modal" data-target="#about" data-title="About" data-content="#about-content">
+                    <li class="nav-item">
+                        <a href="#" data-toggle="modal" data-target="#about" data-title="About" data-content="#about-content" class="nav-link">
                             About
                         </a>
                     </li>
                 </ul>
-                <ul class="nav navbar-nav navbar-right">
-                    <li id="play-pause"><a href="#">Start</a></li>
-                    <li id="step"><a href="#">Step</a></li>
-                    <li id="reset"><a href="#">Reset</a></li>
+                <ul class="nav navbar-nav ml-auto">
+                    <li id="play-pause" class="nav-item"><a href="#" class="nav-link">Start</a>
+                    </li>
+                    <li id="step" class="nav-item"><a href="#" class="nav-link">Step</a>
+                    </li>
+                    <li id="reset" class="nav-item"><a href="#" class="nav-link">Reset</a>
+                    </li>
                 </ul>
             </div><!--/.nav-collapse -->
         </div>
     </nav>
-    <div class="container">
-        <div class="col-lg-4 col-md-4 col-sm-4 col-xs-3" id="sidebar"></div>
-        <div class="col-lg-8 col-md-8 col-sm-8 col-xs-9" id="elements">
+    <div class="container d-flex flex-row">
+        <div class="col-xl-4 col-lg-4 col-md-4 col-3" id="sidebar"></div>
+        <div class="col-xl-8 col-lg-8 col-md-8 col-9" id="elements">
             <div id="elements-topbar">
-                <div class="input-group input-group-lg">
-                    <label class="label label-primary" for="fps" style="margin-right: 15px">Frames Per Second</label>
-                    <input id="fps" data-slider-id='fps' type="text" />
-                    <p>Current Step: <span id="currentStep">0</span></p>
+                <div">
+                    <label class="badge badge-primary" for="fps" style="margin-right: 15px">Frames Per Second</label>
+                    <input id="fps" data-slider-id="fps" type="text">
                 </div>
+                <p>Current Step: <span id="currentStep">0</span>
+                </p>
             </div>
         </div>
     </div>
@@ -59,12 +67,13 @@
         <div class="modal-dialog modal-lg">
             <div class="modal-content">
                 <div class="modal-header">
-                    <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
                     <h4 class="modal-title">About {{ model_name }}</h4>
+                    <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&#xD7;</span>
+                    </button>
                 </div>
                 <div class="modal-body">
                     <div>{{ description }}</div>
-                    <div>&nbsp;</div>
+                    <div>&#xA0;</div>
                     <div style="clear: both;"></div>
                 </div>
             </div>
@@ -73,15 +82,15 @@
 
     <!-- Bottom-load all JavaScript dependencies -->
     <script src="/static/js/external/jquery-2.2.4.min.js"></script>
-    <script src="/static/external/bootstrap-3.3.7-dist/js/bootstrap.min.js"></script>
+    <script src="/static/external/bootstrap-4.6.1-dist/js/bootstrap.bundle.min.js"></script>
     <script src="/static/external/bootstrap-switch-3.3.4/dist/js/bootstrap-switch.min.js"></script>
-    <script src="/static/external/bootstrap-slider-9.8.0/dist/bootstrap-slider.min.js"></script>
+    <script src="/static/external/bootstrap-slider-11.0.2/dist/bootstrap-slider.min.js"></script>
 
     <!-- Script includes go here -->
-	{% for file_name in package_includes %}
+	{% for file_name in package_js_includes %}
 		<script src="/static/js/{{ file_name }}" type="text/javascript"></script>
 	{% end %}
-	{% for file_name in local_includes %}
+	{% for file_name in local_js_includes %}
 		<script src="/local/{{ file_name }}" type="text/javascript"></script>
 	{% end %}
 
diff --git a/setup.py b/setup.py
index 9556834..044566e 100644
--- a/setup.py
+++ b/setup.py
@@ -66,14 +66,14 @@ def ensure_JS_dep_single(url, out_name=None):
 # hardcoded included files and versions in: mesa/visualization/templates/modular_template.html
 
 # Ensure Bootstrap
-bootstrap_version = "3.3.7"
+bootstrap_version = "4.6.1"
 ensure_JS_dep(
     f"bootstrap-{bootstrap_version}-dist",
     f"https://github.com/twbs/bootstrap/releases/download/v{bootstrap_version}/bootstrap-{bootstrap_version}-dist.zip",
 )
 
 # Ensure Bootstrap Slider
-bootstrap_slider_version = "9.8.0"
+bootstrap_slider_version = "11.0.2"
 ensure_JS_dep(
     f"bootstrap-slider-{bootstrap_slider_version}",
     f"https://github.com/seiyria/bootstrap-slider/archive/refs/tags/v{bootstrap_slider_version}.zip",
@@ -92,7 +92,7 @@ ensure_JS_dep_single(
 )
 # Important: when updating the D3 version, make sure to update the constant
 # D3_JS_FILE in mesa/visualization/ModularVisualization.py.
-d3_version = "4.13.0"
+d3_version = "7.4.3"
 ensure_JS_dep_single(
     f"https://cdnjs.cloudflare.com/ajax/libs/d3/{d3_version}/d3.min.js",
     out_name=f"d3-{d3_version}.min.js",

diff --git a/tests/test_batch_run.py b/tests/test_batch_run.py
index 072b1da..67ce3bc 100644
--- a/tests/test_batch_run.py
+++ b/tests/test_batch_run.py
@@ -18,6 +18,8 @@ def test_make_model_kwargs():
         {"a": 1, "b": 0},
         {"a": 1, "b": 1},
     ]
+    # If the value is a single string, do not iterate over it.
+    assert _make_model_kwargs({"a": "value"}) == [{"a": "value"}]
 
 
 class MockAgent(Agent):

Migrate D3 from v4 to v7
The current D3 version that is [used in Mesa](https://github.com/projectmesa/mesa/blob/main/mesa/visualization/templates/js/d3.min.js), version [4.13.0](https://github.com/d3/d3/releases/tag/v4.13.0), is almost 4 years old. Three new major versions have been released since. Version 5.0 and 7.0 contain relatively few breaking changes, but 6.0 contains a bit more.

- [Changes in D3 5.0](https://github.com/d3/d3/blob/main/CHANGES.md#changes-in-d3-50)
- [D3 6.0 migration guide](https://observablehq.com/@d3/d3v6-migration-guide)
- [v7.0.0 breaking changes](https://github.com/d3/d3/releases/tag/v7.0.0)
tests/test_batch_run.py::test_make_model_kwargs
tests/test_batch_run.py::test_batch_run tests/test_batch_run.py::test_batch_run_with_params tests/test_batch_run.py::test_batch_run_no_agent_reporters tests/test_batch_run.py::test_batch_run_single_core