Grain/texture slider for the background

[morceaudebois]

The Arc Browser has a really cool grain/texture knob for customizing its UI and make it look like frosted glass. I think we could try something similar in Bonjourr's backgrounds, added to the blur it might look really nice.

[victrme]

No idea how they do it, but we need it... Related to #122 !

[morceaudebois]

I think it would just be a .png texture file used as a background. Then the slider would change its opacity. This is the way I do it on my website anyway, check out the #bg-grain element!