Refine Scriptler run/edit form styling for modern Jenkins themes

[BobDu]

After:

Before:

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 demonstrate the feature works or the issue is fixed

[mtughan]

This looks great, @BobDu! This had been on my to-do list for a while, but hadn't gotten around to it. Thanks for tackling this!

Just a few small things I'd like to see addressed if possible before I approve this.