fix(css): improve look of tables in panel

[nijel]

Proposed changes

• make table heading (reverts part of proposal(ui): modernize panel and table styles #12982)
• use consistent border inside the tables
• avoid double border in headings

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information

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After: