CSS always drives me up the wall. I guess that's why I just concentrate on functionality and leave the appearance to someone else.
One approach, that sometimes works, is to right click on the element you are interested in and "Inspect Element" (different but similar terms on different browsers). Then navigate up and down the structure in the developer window, noting what tc- classes are used. Then manually try over-riding the classes to see what you can change. When you get something to work, you can try it in a stylesheet tiddler. If that doesn't work, it usually means you have to notch up the specificity, so that your custom style gets applied.
Good luck!
-- Mark