Improve artifact list readability in dark theme

[zbynek]

Currently the artifact list (for sufficiently many artifacts) looks wrong in dark theme:

This PR makes the background transparent

Proposed changelog entries

• Improve artifact list readability in dark theme.

Proposed upgrade guidelines

N/A

Submitter checklist

• [n/a] (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
• [n/a] Appropriate autotests or explanation to why this change has no tests
• [n/a] For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@fqueiruga @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).

[zbynek]

CC @janfaracik in case this is conflicted with your UI cleanup

[janfaracik]

CC @janfaracik in case this is conflicted with your UI cleanup

LGTM :)

[daniel-beck]

(FTR this is shown between 16 and 40 files)