Fixes: https://github.com/go-gitea/gitea/issues/29981. Introduce
`.secondary-nav` as a universal way for styling and margin adjustments
inside `.page-content`.
If the first child of `.page-content` is `.secondary-nav`, we add margin
below it, otherwise we add padding to the first child. Notable changes:
- `--color-header-wrapper` is replaced with `--color-secondary-nav-bg`.
- `navbar` class is removed.
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Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
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