Improve accessibility and clean up components

[janfaracik]

Starting this with an apology for the amount of SCSS changes, it's a little painful to read, the majority of the changes are swapping to variables or cleaning things up. If splitting this out further is necessary (such as I've done for #10182 #10176 #10175 #10174) I'm happy to do so.

---

This PR aims to improve accessibility of our components and also tidies them up a little. This is quite vague so I'll do my best to demonstrate with pictures:

Controls now have a subtle border to improve contrast:

This isn't major, but it should help contrast a little bit across Jenkins. Components now lift off the page a little more than they used to, and this is applied consistently across buttons, menus and more.

The 'Refresh' button on the 'Plugin updates' page, note the button and the banners consistent borders.

---

Control heights are now consistent at 38px, focus states are more compact:

For example, a side panel task link is the same height as a button, which is the same height as a search bar. This keeps things nice and tidy, and keeps app bars consistent in height across pages.

Before

After

---

Prefers increase contrast is now respected in Jenkins:

When enabled the vast majority of components will show a hard, contrasty border that separates them from their backgrounds.

---

Button classes have been cleaned up, making use of relative color syntax and variables:

Inspired by recent works in the buttons file, this change aims to make such changes easier in the future and greatly simplify one of our most used components.

Testing done

• Compared each page on Design Library, changes are expected/no breakages

Proposed changelog entries

• Improve accessibility and clean up components

Proposed upgrade guidelines

N/A

Submitter checklist

Preview 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

@jenkinsci/sig-ux

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

Maintainer checklist

Preview 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).

[A1exKH]

LGTM.

[timja]

Tested locally LGTM

---

/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!

[timja]

Tested locally LGTM

---

/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!