Starting from near-zero
To make a Tiki theme that is not derived from an existing theme, a good way is to start with a "blank" CSS file. This way, it will be unique, not "inheriting" any page treatments from another theme. The problem with starting from zero, though, is that it isn't clear what elements need to be styled. As a practical matter, it would probably be a good idea to take one of the more lightweight new theme stylesheets and just delete or replace the values, retaining the selectors (divs, ids, and spans, etc.).
Older information
Note: The rest of the information on this page is out of date and here for historical purposes only.
Tikiwiki2_CSS_selectors is a now-outdated document that shows all — or nearly all — the selectors found in Tiki (at that time) but only has placeholder color properties and some basic layout properties that probably don't need to be changed. The color properties were added to help prevent contrast problems, and each can be replaced en masse by the new theme color. In the final version, comments and unused selectors can be deleted to reduce the file size if desired. Copy this CSS, give it a new theme name, and the new "empty slate" theme is under way.
This file has selectors for both a "litecss" liquid layout, and a table-based layout for a "fixed width" theme (as demonstrated on Themes_for_TW2 ). Use the appropriate template files, if necessary.
If the new look can be created using the CSS file alone (and perhaps background graphics), then no template modification is needed. More complex layout changes may require theme-specific templates as well. Templates in Theme-making introduces template modification in theme-making.