Allow one theme to extend from another

[hipstersmoothie]

What Changed

See title

Why

A lot of themes can be expressed as sub themes

Todo:

• [x] Add tests
• [x] Add docs

Published PR with canary version: 2.3.0-canary.42.534

[hipstersmoothie]

:rocket: PR was released in v2.3.0 :rocket: