Styling can be hairy, above all across browsers, but once you get a
grip on the how-to's it get's less and less complicated, especially
the basics. So, if you haven't yet, first get FIREBUG for Firefox.
Then inspect elements rendered by the browser (like tabs) as to what
they consist of... you will find css classes, like...
.tabSetWrapper
.tab
.tabSelected
.tabUnselected
.tabContents
etc... which give you an idea of the actual html structure underlying
to tabs
Knowing this, and that you may have different areas for which you want
to style tabs differently,like
.viewer
#tiddlerDisplay
#sidebar
#mainMenu
...you can put stuff into your Stylesheet, like
.viewer .tab {border: 3px solid #999;-moz-border-radius:3px; -webkit-
border-radius:3px;background:#eee;}
#sidebar .tabContents .externalLink {text-decoration:none;border-
bottom:1px dotted #eee;}
For more examples, you could sneak into the StyleSheet as part of
[[tbGtdTheme]] of my tbGTD
http://tbGTD.tiddlyspot.com
There you find .tab style-definitions especially for the sidebar and
those vertical tabs (which I use for my help-section) which I created
recently together with Eric Schulman...
http://verticaltabs.tiddlyspot.com
Here's some viable information with examples on css...
http://www.w3schools.com/CSS/