Screenshots for Themes

[Penaz91]

This is an idea that has been bouncing in my head for a bit: including a single screenshot for each theme in the themes folder.

This will definitely make the repository heavier to download, but it should give us the possibility to have an always-up-to-date theme gallery on https://libresplit.org by periodically pulling from this repository or do it on-demand.

[wins1ey]

Could just add a readme to the themes directory with images instead of storing them in the repo.