Surely, the base theme should not be making an assumption like 'centre everything', which is one of the things that makes theming Moodle 1.9 a real pain (if you want a left-aligned theme).
The participants table is a large table that should take up at least 85% of the width of the content and have equal space to the left and right of the theme. The current base theme does not set an alignment and does not set a width for this table. Therefore, it has to be set in the new theme.
I can go about this in one of multiple ways:
1.) I can add such common layout styles directly to the Base core.css (or otherwise appropriate) stylesheet.
2.) I can create a new theme (base+) that is based on the Base theme and build ALL of my themes off that (this seems like a strange choice to me but will be necessary considering the amount of coding necessary to get base ready to actually work as a starting point for projects).
3.) I can create new stylesheets for these new layout elements. That way, if a theme developer wants to exclude my centering (or other elements) they can exclude them from the styles.