Currently tables of data in Moodle aren't very themeable, due to a variety of bugs and issues. This bug hopes to serve as a central place to track progress on various smaller issue that combine to make this problematic.
The ideal situation would be:
a small number of lines (say <5) of CSS can consistently control the look of every presentation of standard tabular data across Moodle, including those generated by 3rd party plugins, and be easily tweaked by themers for different looks without accidentally affecting complex tables (gradebook etc.), without affecting user-generated-content (which will often use tables for layout), and without affecting the tables still used for layout in Moodle itself.
Getting there involves solving a web of related issues, which mostly deserve bugs of their own.