You make the CSS and branding into a thing that folks are "discouraged" from distributing. That's most of the WP plugin and theme ecosystem, or was when I retired from it.

Prevent anyone else from profiting off of the ecosystem.

Wordpress might technically be GPL, but the code itself routes most of the key features of Wordpress through Automattic.

If you have a massive technical moat around the ecosystem, it would make sense to be against erecting walls.