Update appearance of controls in header

[janfaracik]

• Uses the item mixin for header links, this provides consistent rounding/hover/focus states
• Use variables for colours, remove old hardcoded colours
• Refined the appearance and animations of the warning links/dropdowns to be inline with modern dropdowns

Before

After

Testing done

• Links works as before

Proposed changelog entries

• Update appearance of controls in header

Proposed upgrade guidelines

N/A

Submitter checklist

Beta Give feedback

1. The Jira issue, if it exists, is well-described.
2. The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
3. There is automated testing or an explanation as to why this change has no tests.
4. New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
5. New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
6. New or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
7. For dependency updates, there are links to external changelogs and, if possible, full differentials.
8. For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@mention

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

Maintainer checklist

Beta Give feedback

1. There are at least two (2) approvals for the pull request and no outstanding requests for change.
2. Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
3. Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
4. Proper changelog labels are set so that the changelog can be generated automatically.
5. If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
6. 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).

[timja]

/label ready-for-merge

---

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!

[mawinter69]

That change caused a regression.
How to reproduce:

1. Have some admin warnings
2. Go to /computer
3. Click to show the admin monitors
4. Click again to hide them

-> The buttons to add new Node and configure monitors are not working anymore as they are hidden behind the div with z-index 1000 and opacity: 0
That div should be hidden with display: none

[daniel-beck]

@mawinter69 I filed that as JENKINS-72679 and linked #8941.