As discussed in
https://github.com/go-gitea/gitea/pull/24953#issuecomment-1565630156.
## ⚠️ BREAKING ⚠️
1. The `ui.THEME_COLOR_META_TAG` setting has been removed. If you still
need to set the `theme-color` meta tag, add it via
`$GITEA_CUSTOM/templates/custom/header.tmpl` instead.
2. The non-standard `default-theme` meta-tag added in
https://github.com/go-gitea/gitea/pull/13809 has been removed. Third
party code that needs to obtain the currently loaded theme should use
the `theme-<name>` class on the `<html>` node instead, which reflect the
currently active theme.
