Atahualpa is a very customizable and browser-safe (incl. IE6) theme: Choose between fixed or flexible width layout, with min/max width, set between 0 and 4 sidebars on a per-page basis, create custom widget areas, import and export styles (several styles included), choose colors with pickers, adjust over 200 options.
Atahualpa is #1 in total downloads and maintained since 2007.
1. Go to Appearance -> Themes.
2. Click on Install Themes at the top.
3. Click on Upload tab.
4. Upload zip archive file via web interface.
5. Activate newly installed theme.