fix background of spacing select in dark theme

[mawinter69]

fixes #372

background hover effect is replaced by a border hover effect
background color is that of regular Jenkins selects

Before:

After:

Testing done

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[janfaracik]

LGTM :)