Add a new dark theme based on Snow theme

[florealcab]

I changed the Squares Dark theme to adapt it to the snow theme (just changed the background color).

But I also made changes in the dark theme:

• Make background less dark to have less contrast
• Fix menus background to be dark, not white
• Remove linear gradients on top and bottom: use solid color instead

It match to my tastes, but maybe to some other users, si I share it, maybe we can add it to this repo so?

PS: thank you for forking rainloop and add a dark theme and so many wonderful new features, this app has reborn :)