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>

CHANGELOG.md

@@ -22,6 +22,9 @@ You should also include the user name that made the change.
 - update dependencies @syuilo
 - enhance: display URL of QR code for TOTP registration @syuilo
 - make CAPTCHA required for signin to improve security @syuilo
+- The theme color is now better validated. @Johann150
+  Your own theme color may be unset if it was in an invalid format.
+  Admins should check their instance settings if in doubt.
 - Perform port diagnosis at startup only when Listen fails @mei23
 
 ### Bugfixes