Add dark mode to the default CSS

[bartekpacia]

Basically the title. Would be nice to have it.

[maciejzj]

I thought that it would be nice too.

However I would also like to move the themes from a separate repository (https://github.com/maciejzj/ssb-themes) into this one. I believe merging these two repos and adding a dark mode can be done in one shot.

Will do this in some time.

[bartekpacia]

In the meantime, I made a dark theme myself for my website -> https://garden.pacia.tech

source code: https://github.com/bartekpacia/garden

When I have some time I'll extract it and contribute either to this repo or ssb-themes.

[maciejzj]

I've integrated the themes into the main repo. Feel free to submit as many themes as you want :)