enhance: uniform theme color (#8702)

* enhance: make theme color format uniform

All newly fetched instance theme colors will be uniformely formatted
as hashtag followed by 6 hexadecimal digits.

Colors are checked for validity and invalid colors are not handled.

* better input validation for own theme color

* migration to unify theme color formats

Fixes theme colors of other instances as well as the local instance.

* add changelog entry

Co-authored-by: syuilo <Syuilotan@yahoo.co.jp>