For more control over style, try the StylePlugin (also available at twiki.org).
A quick tutorial is located at StylePluginDetails. The VoidSkin controls the overall layout of the page. The StylePlugin gives you control over the style of individual paragraphs and text. It's completely configurable and uses only CSS styling to achieve its effects.
The default skin on my TWiki site is the 'void' skin, so named because the stylesheet for it was derived from one at http://www.dotvoid.com (with permission). The skin uses almost no graphics - just a small gif for the "Go" button next to the search box at the top of the page, and a "preview" graphic on the preview page. Layout is entirely controlled by css - i.e. the skin does not use tables, frames, or iframes. Consequently, the skin loads quickly and displays well in browsers that support CSS. I've tested the skin with all versions of IE 5.0 and above, Mozilla 1.0, and Konqueror. It looks about the same in each of those browsers.
User-selectable settings let you choose from among several color schemes. And you can select either fixed-size or variable-size fonts (the latter allow the user to use the browser to change the displayed font size).
All templates are included - including the 'oops' and search-result templates.
See VoidSkinDev for bug reports and upcoming release information.
VoidSkin.zip: Snapshot of voidskin files plus twiki bin and lib
You can see what the various color schemes and the 2 sizing methods look like - select a color or size from the dropdowns (it only takes effect while you're still on this page - and only one at a time).
unzip and edit the file twikidata/TWiki/TWikiPreferencesForVoidSkin.txt and adjust the settings there to fit your needs, and add those settings to TWikiPreferences.
As described under Installation, you will need to add these to TWikiPreferences and edit to taste. You really should edit the file TWikiPreferencesForVoidSkin.txt - the settings are shown here just for documentation purposes: