Document appearance settings

[orangesunny]

Describe the issue

Documentation lacks info about appearance customization.

I already tried

Describe the steps you tried to solve the problem yourself.

• I've read and searched the docs and did not find the answer there.
  If you didn’t try already, try to search there what you wrote above.

Expected behavior

A short description about Weblate appearance customization options

[nijel]

I've just added a note on this in 4d6c2b6, more docs is really needed here.

[github-actions]

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