Re: Global theme set?
A theme is already being applied globally to your site (as set in Admin > General). It is global in the sense that it is applied to all Tiki content. The line of code you quoted is just the one that finds the theme set by Admin or the user to be applied.
I'm not sure what you mean by superceding sections, categories, objects, etc. These are styled within each theme's CSS file and thus can vary or be the same depending on how the theme author chose to do it. If you want consistent treatment for these things, you need to find the theme that comes closest to doing that or else make a new theme (stylesheet) that does apply properties to all selectors to produce a consistent, global look, etc.
-- Gary - themes.tw.o