Move non-theme-specific inline editor styles into core.

Fix for #8571 (targeting release branch).

@larz0 can you take a look?

@mackenza does this do the trick for you?
@larz0 You mean with that selector removed?  If it looks fine with it removed, want to submit a tiny cleanup PR to make that change?  (I guess targeted at master -- no point in squeezing it onto the release branch).