Customize scrollbar in Mozilla Firefox

[zefr0x]

This uses the CSS's scrollbar-color to change the colors of the scroll bar. For now it's only supported in Firefox.

For Chromium ::-webkit-scrollbar is supported, but it's not a standard yet, and there may also be a large incompatibilities between implementations...

Before:

After:

[unixfox]

What's the result on the white theme?

[zefr0x]

I uploaded new images with the light theme 👍 By the way, my taste in colors is bad so I'm not sure about them. Someone might have better colors to be used.

[unixfox]

Please squash your commits into one.

[SamantazFox]

I'm not super convinced about the utility of that PR.

IMHO, we shouldn't change UI elements that belong to the browser, especially when the option to do that is barely supported.