### What problem does this PR solve?
For proper semantics Layout should use HTML `<main>` element to wrap the
Header and Outlet which produce`<section>` HTML elements.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
Feat: Replace color variables according to design draft #3221
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
Feat: Modify the style of the home page in bright mode #3221
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
Feat: Add dataset sidebar #3221
### Type of change
- [x] New Feature (non-breaking change which adds functionality)