Block themes should inherit their own styles in Learning Mode

[donnapep]

Is your feature request related to a problem? Please describe

When using a block theme other than Course, the Learning Mode styles are overridden to ensure that LM looks (mostly) the same across all themes. Perhaps we shouldn't be this opinionated, and should inherit from the block theme's styles instead. (This may be worth further discussion, as it will impact existing sites.)

Describe the solution you'd like

Don't override block themes' styles in Learning Mode. The only styles that should be overridden are those of non-block themes like Divi and Astra.

Consideration: Should we only do this for new installs? Because otherwise the look & feel of Learning Mode will suddenly change on sites using block themes, and perhaps not for the better.

Also, we probably shouldn't do this until the other issues with global styles and styles in general in Learning Mode are resolved.