Conflicts:
- `app/lib/themes.rb`
- `app/views/layouts/application.html.haml`
- `app/views/layouts/embedded.html.haml`
- `app/views/layouts/error.html.haml`
- `config/settings.yml`
All these conflicts are because glitch-soc and upstream have different theming
systems and upstream changed a few things to have dynamic theme selection based
on system settings.
Conflicts were solved to take that into account, and `current_theme` has been
changed in the process to return a tuple of `[flavour, skin]` to be used in
the `theme_style_tags` helper.
