[JENKINS 62750] Transform rss bar items into transparent buttons

[fqueiruga]

See JENKINS-62750.

This PR reworks the RSS bar to make the links transparent buttons. It also makes the items more accessible by combining the icons and text into a single link.
As a side effect, this PR also adds theming support for transparent buttons, thus opening core support for jenkinsci/dark-theme-plugin#105.

Screenshots

Home screen

Log recorders

Proposed changelog entries

• Entry 1: JENKINS 62750, Restyling and improved accessibility for the RSS feed bar.
• ...

Proposed upgrade guidelines

N/A

Submitter checklist

• (If applicable) Jira issue is well described
• Changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developer, depending on the change). Examples
  • Fill-in the Proposed changelog entries section only if there are breaking changes or other changes which may require extra steps from users during the upgrade
• Appropriate autotests or explanation to why this change has no tests
• For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@timja

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least 2 approvals for the pull request and no outstanding requests for change
• Conversations in the pull request are over OR it is explicit that a reviewer does not block the change
• Changelog entries in the PR title and/or Proposed changelog entries are correct
• Proper changelog labels are set so that the changelog can be generated automatically
• If the change needs additional upgrade steps from users, upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the PR title. (example)
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[fqueiruga]

This will enable theming support for the background

[timja]

Changes here look good, tested locally

Is the rss feed design intentionally not updating the log recorders page?

http://localhost:8080/jenkins/log/all

[fqueiruga]

I wasn't aware of that page, do you think it's worth updating in the same way?

[timja]

not sure if anyone even uses it tbh...

If it doesn't take long would be good to be consistent

[fqueiruga]

Done, updated the screenshots

[timja]

Thanks for the PR!

This PR will be merged after 24 hours if there's no negative feedback