feat: add theme as regular setting COMPASS-6067 COMPASS-5284

[addaleax]

• Modify and read theme as a regular preference value, and use it to set the Electron theme settings instead of using custom listeners and IPC ping-pong for changing its value.
• Add the leafygreen darkmode flag to the preferences list.
• Add an entry to the settings modal for changing the selected theme.

Description

Checklist

• [x] New tests and/or benchmarks are included
• [ ] Documentation is changed or added
• [ ] I have signed the MongoDB Contributor License Agreement (https://www.mongodb.com/legal/contributor-agreement)

Motivation and Context

• [ ] Bugfix
• [ ] New feature
• [ ] Dependency update
• [x] Misc

Open Questions

Dependents

Types of changes

• [ ] Backport Needed
• [x] Patch (non-breaking change which fixes an issue)
• [ ] Minor (non-breaking change which adds functionality)
• [ ] Major (fix or feature that would cause existing functionality to change)