Default CSS group should be CSS_DEFAULT

> It's funny that if you leave `group` undefined, Backdrop will assign this file a `CSS_SYSTEM` weight (-100), rather than `CSS_DEFAULT` which would be more logical.
> 
> Sounds like a follow-up issue. 

 _Originally posted by @izmeez in [#7144](https://github.com/backdrop/backdrop-issues/issues/7144#issuecomment-4564903434)_