I feel your pain. GWT uses Google closure stylesheets library but that library has been archived by Google in favor of pure JS solutions like PostCSS. The GWT SDK could be updated to the very latest version of closure stylesheets but there would still be a lot missing. However you would be able to use css variables (although I think you could not use @property for detailed definition).
In general the closure stylesheets library uses JavaCC to generate a parser from a grammar. So it is not super terrible to update it if someone has JavaCC experience. But personally I think CSS is moving pretty fast and a more flexible solution would be better, so you can play with new CSS features without waiting for someone to update a grammar. Even though I created a local project to explore some ideas nobody is actively working on improving the situation with CssResource.
With regard to bounties: So far there hasn't been any as far as I know. But of course you can hire someone to fork and update closure stylesheets and then let us know about the fork. If you plan to do that it might be a good idea to make sure its not an AI agent who effectively wrote the code (unclear license issues). In the issue
https://github.com/gwtproject/gwt/issues/10036 is a link to a closure stylesheets fork which did some continued development so maybe the maintainer would be a good candidate to ask. There is also the company Vertispan which can be paid for GWT related services.
-- J.