Styling improvements

[filiptronicek]

See commits for exact details on everything changed, but most notably:

• fixed the fact styles wouldn't reflect locally when you change them (https://github.com/Meziklassociation/meziklasi/commit/d896a1d82c33875c15aa5f1e5a120ec5c6cac3ed)
• added dark mode (activated by preferes-color-scheme)
  • There's now a invertable image class, which enables purely black images to be white in dark mode (see /o-vesnici/ or screenshot below)

[yagarea]

Mega fancy