How to change border color of Bootstrap-select
<p>For a custom framework like Bootstrap select <a href="https://silviomoreto.github.io/bootstrap-select/" rel="nofollow">https://silviomoreto.github.io/bootstrap-select/</a> how do I change the border color of a select box in Javascript? </p>

<p>I tried this but does not work. </p>

<pre><code>document.getElementById("box").style.borderColor = "red";
</code></pre>

<p>Javascript</p>

<pre><code>function validate() {
    var ok = true;

    var box = document.getElementById("box").value;

    if (!box) {
        document.getElementById("box").style.borderColor = "red";
        ok = false;
    } 

    return ok;
}
</code></pre>

<p>HTML</p>

<pre><code>&lt;form action="#" onsubmit="return validate()" method="POST"&gt;
    &lt;select id="box" name="num" class="selectpicker form-control"&gt;
      &lt;option value="" selected="selected"&gt;select number&lt;/option&gt;
      &lt;option value="1"&gt;1&lt;/option&gt;
      &lt;option value="2"&gt;2&lt;/option&gt;
    &lt;/select&gt;

    &lt;div class="text-center"&gt;
        &lt;button type="submit" class="btn btn-primary"&gt;submit&lt;/button&gt;
    &lt;/div&gt;
&lt;/form&gt;
</code></pre>