Initial support for #25680
This PR only adds some simple styles from GitHub, it is big enough and
it focuses on adding the necessary framework-level supports. More styles
could be fine-tuned later.
Part of https://github.com/go-gitea/gitea/issues/27097:
- `gitea` theme is renamed to `gitea-light`
- `arc-green` theme is renamed to `gitea-dark`
- `auto` theme is renamed to `gitea-auto`
I put both themes in separate CSS files, removing all colors from the
base CSS. Existing users will be migrated to the new theme names. The
dark theme recolor will follow in a separate PR.
## ⚠️ BREAKING ⚠️
1. If there are existing custom themes with the names `gitea-light` or
`gitea-dark`, rename them before this upgrade and update the `theme`
column in the `user` table for each affected user.
2. The theme in `<html>` has moved from `class="theme-name"` to
`data-theme="name"`, existing customizations that depend on should be
updated.
---------
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>
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.
