In Tiki 3.0, all of the bundled themes are full-width, liquid layouts that use the *litecss method of floating divs for the page's content columns. Of these themes, Strasa and Coelesce are a little more complex due to their unique site header and blog and forum post treatments. Darkroom, Feb12, TheNews, and Tikinewt and somewhat simpler so are probably easier starting points. Tikinewt is the only theme in Tiki 3.0 that imports layout.css, which is the way of the future. The other bundled themes still have all the layout properties in their stylesheets.
If your design isn't full-width, but rather the content is in a "fixed-width" area in the page center with margins on the sides (and maybe on top and bottom), it may be possible to achieve this using the state-of-the-art *litecss method modified with a few lines in the CSS file. See Fixed-width layout without a table for more information.
Otherwise, a traditional table can be used for layout. None of the bundled themes use this method, to facilitate theme maintenance in the core code. But a number of "aftermarket" themes at mods.tikiwiki.org do; these include Andreas08, Dark Shine, Eatlon, Kubrick, and so on. Using these Tiki3.0-compatible themes as a model is encouraged.
In TikiWiki 2, theme-related files were in a transition period, moving away from old-school table-based layout and toward liquid layout method based on divs. Themes using the new method include, in the Tiki 2 package, Darkroom, Feb12, The News, and Tikinewt, and the themes on Themes_for_TW2 (the "fixed-width" themes of which use a table for layout due to IE7's lack of support for the display:table CSS property). The focus of these themes has been on more modern and economical use of CSS, so any would be a good model for a custom theme. It's best to avoid basing a custom theme on an older theme.