Missing Style for overlay in base-theme

[B34v0n]

When using the Component-Overlay, on the example site it looks just fine.
But when I include it and use the base-theme from here, it's not visible and the background is not overlayed behind the "dark part".

I analysed the CSS on the example site and found class, that is set there but not here:

.p-component-overlay {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}

I manually included it and it works just fine.
Can that please be included in the base-theme?