* 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>
