It seems that there is no readily available solution. So if I were to create one, it would go something like this:
href="theme-base-renegade.css"
theme-base-renegade would style all of the default markup in its own styleish little way. Whatever that may be: Super padded with big fonts and so on, or incredibly tiny with minimal line-height, font-size, etc..
href="theme-color-grayhaze.css"
theme-color-darkness would specifically color the default markup. Things like th, tr, td, dl, dt dd, etc, would have be darker/brighter than others with their respective font-colors and such to complement the element background-colors.
Since both stylesheets would be styling the default markup - not specific to styles or id's - the two stylesheets could be swappable with other like type themes components and not interfere with one another.
After that.. it's CSS! You could overwrite any one of the many styles used to create that theme. Plus, with all of the tools we have to debug, overriding otherwise desirable styles in order to apply specific/custom ones would be much more convenient.