A Quick dump of something that could be added to the Doco.
The Tiddler
Using Stylesheets on TiddlyWiki.com
https://tiddlywiki.com/#Using%20Stylesheets
Should include information on the following
Control Panel > Info > Advanced > Stylesheets
Shows the order of StyleSheets, and using the tag pill therein you can drag and drop the order to reorder them.
CSS by definition is Cascading if you can't see what the cascade order is, you can easily be tricked
Unless we were to provide details of all the class names already applied to all TiddlyWikis elements we should also provide information that allows people to identify those elements such as via inspect. And suggest how these can be over ridden. We do not need to teach css only say how TiddlyWiki deals with it, and a few high level pointers.
Regards
Tony