Fixes: https://github.com/go-gitea/gitea/issues/36543
When running `make watch`, the backend may start before webpack finishes
building CSS theme files. Since themes were loaded once via sync.Once,
they would never reload, breaking the theme selector and showing a
persistent error on the admin page.
In dev mode, themes are now reloaded from disk on each access so they
become available as soon as webpack finishes. Production behavior is
unchanged where themes are loaded once and cached via sync.Once.
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
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.
