The new starterkit theme doesn't rely on classy theme anymore and it isn't added as a contrib theme anymore in the rocketship distribution.
However, the styling in lb.css is still relying on classes & markup that was previously added by classy. (templates, classy styling, ...)
Start a new project and check the media library inside a layout builder modal, eg, on the photo gallery block
Adjust the styles in lb.css in a way that is backward compatible with older projects where the theme still relies on classy
Needs review
6.2
Code