Followup of #23876 according to my unreleased review demanding tooltips.
Additionally
- add a `muted` equivalent for buttons
- convert `switch to legacy` to an actual button
- enroll `switch to legacy` in the builtin pseudo focus cycle
- remove spaces between the buttons
The effect of the `muted` class is what you would expect: The button
loses all of its normal styling, and is defined only by its content instead.
This will help reduce a11y infractions in the future, as that was one of
the major points why people didn't use `<button>` tags and decided on a
bad fix (i.e. through `<div>`s) instead.
## Appearance

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Co-authored-by: silverwind <me@silverwind.io>
Caught by @wxiaoguang in
https://github.com/go-gitea/gitea/pull/23337#issuecomment-1467317742.
Additionally, there were three instances that have the same content as `templates/base/deletion_modal_actions.tmpl` but that are not intended to delete something.
Instead of renaming the template above, these instances were simply re-hard-coded again.
Renaming/improving the template above is left for future PRs.
At the moment, this is only used to replace the color of the `viewed`
checkbox and of the `has changed` label.
Previously, the used variable accentuated always either darker or
lighter, which meant that one theme looked good while the other didn't.
Co-authored-by: silverwind <me@silverwind.io>