feat: dark mode support on Rill cloud

[briangregoryholmes]

Checklist:

• [ ] Covered by tests
• [ ] Ran it and it works as intended
• [ ] Reviewed the diff before requesting a review
• [ ] Checked for unhandled edge cases
• [ ] Linked the issues it closes
• [ ] Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• [ ] Intend to cherry-pick into the release branch
• [ ] I'm proud of this work!