The point of this PR is to make it easier to implement scrollbars (#683).
With this PR, the themes move from a body tag-based system (e.g. `body.theme-scarlet`) to a system where they simply declare global CSS and we use CSS specificity order to give us the right theme.
* more attempts to fix test flakiness
* remove testcafe roles entirely
* really remove testcafe roles
* run testcafe in parallel x2
* run testcafe in parallel x4
* fix online/offline forcing in tests
* fix pin test