We're actually experimenting with deprecating access to classes (i.e exposing components rather than class names as the public API) - will be interesting to see how popular that approach is.
2. LESS has been our favourite pre-processor for a white but we're finding it doesn't play well with packaging modular components on npm. Some real limitations around the @import logic, and the core team refuse to make it to work with node-style require path resolution which is disappointing.
From here I think it's more likely we go with the "styles in components" approach (via Radium or similar) and make variables / themeing available via context. Much easier to package & use "out of the box". Might keep a "base" css include though, or possibly use the Bootstrap 4 "Reboot". The distinction between, and best way to package, component styles and page styles is something we're putting a lot of thought into!