CSS transition does not show when there's a background-color
<p>I made a modal and it does not do the transition effect when I apply a background-color to my #main div which contains all the content, after removing the background the modal does do the transition effect and disappears slowly.</p>

<p>To ge the modal just click on register on the top navbar.</p>

<p>Live demo for the modal along with the background-color on #main:
<a href="http://79.179.201.217/" rel="nofollow">http://79.179.201.217/</a></p>

<p>Live demo for the modal long without the background-color on #main:
<a href="http://79.179.201.217/test.php" rel="nofollow">http://79.179.201.217/test.php</a></p>

<p>Just to note that the following under #main causes the issue.
background-color: #eee;</p>

<p>CSS:</p>

<pre><code>#main {
    width: 85%;
    max-width: 875px;
    margin: 70px auto;
    border-radius: 5px;
    padding: 15px;
    z-index: -600;
    background-color: #eee;
}

.modal {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: rgba(0, 0, 0, 0.4);
    z-index: -500;
    transition: opacity 0.18s linear, transform 0.18s linear;
    transform: scale(1.4);
    opacity: 0;
}

.modal-close {
    display: block;
    color: #555;
    font-size: 13px;
    text-decoration: none;
    text-transform: uppercase;
    position: absolute;
    top: 6px;
    right: 20px;
}

.modal-content {
    display: block;
    margin-top: 10px;
}

.modal-container {
    display: table;
    margin: 220px auto;
    position: relative;
    border: 1px solid #eee;
    background-color: #eee;
    padding: 25px;
    border-radius: 3px;
    box-shadow: 0 1px 10px 0 rgba(0, 0, 0, .5);
}

.modal-container.x2 {
    width: 80%;
    max-width: 200px;
}

.modal-container.x3 {
    width: 80%;
    max-width: 300px;
}

.modal-container.x4 {
    width: 80%;
    max-width: 400px;
}

.modal-container.x5 {
    width: 80%;
    max-width: 500px;
}

.modal-container.x6 {
    width: 80%;
    max-width: 600px;
}

.modal-container.x7 {
    width: 80%;
    max-width: 700px;
}

.modal-container.x8 {
    width: 80%;
    max-width: 800px;
}

.modal-header {
    margin-bottom: 20px;
    border-bottom: 1px solid gray;
    font-size: 18px;
    color: #555555;
    font-weight: 500;
    text-transform: uppercase;
}

.modal.modal-visible {
    opacity: 1;
    z-index: 1000000;
    transform: scale(1.0);
}
</code></pre>