Add basic themes documentation

[bartaz]

Done

Updates some legacy themes documentation.

Fixes https://warthogs.atlassian.net/browse/WD-7593

QA

• Review updated documentation:
  • Check new Themes page: https://vanilla-framework-5378.demos.haus/docs/base/themes
  • Check updated colour settings (with deprecated theming section): https://vanilla-framework-5378.demos.haus/docs/settings/color-settings#color-theming
  • Make sure that old paper background page redirects to new themes page: https://vanilla-framework-5378.demos.haus/docs/base/paper

Check if PR is ready for release

If this PR contains Vanilla SCSS code changes, it should contain the following changes to make sure it's ready for the release:

• PR should have one of the following labels to automatically categorise it in release notes:
  • Feature 🎁, Breaking Change 💣, Bug 🐛, Documentation 📝, Maintenance 🔨.
• Vanilla version in package.json should be updated relative to the most recent release, following semver convention:
  • if CSS class names are not changed it can be bugfix release (x.x.X)
  • if CSS class names are changed/added/removed it should be minor version (x.X.0)
  • see the wiki for more details
• Any changes to component class names (new patterns, variants, removed or added features) should be listed on the what's new page.

Screenshots

[webteam-app]

Demo

Jenkins

demos.haus