Board Thread:Support Requests - Designing Your Wiki/@comment-5431054-20130510003236/@comment-4674838-20130510054242

It is not possible to do it for a category, as there is no way to deploy CSS on a per-category basis (without the use of JavaScript).

It can be done on a per-category basis with JavaScript, but the downside is that JavaScript takes a little time to take effect. What will happen is the wordmark will be normal for a second when the page first loads, and then you will see it switch to the new wordmark once the JavaScript takes effect.

Like you mentioned, another option would be to put the CSS in a MediaWiki page and verbatim it on every single page you want to have the different wordmark. Not exactly the best idea, but yes, it would work.