One of the largest complains to LibreOffice has been its dated look. Given that time will always make something look dated, the developers probably thought that they should abstract the look to CSS or something and then make their lives easier down the line. The added benefit is now they can tell people that don't like the "dated" look to pick one of many.
Whilst you might want a single clean look, you definition and someone elses might differ.
Also, it's probably easier to get a designer to improve the default theme (or make a new default theme), than to patch possible improvements into the core (when there is no theme engine).
Separation of concerns is always a good thing.
edit: seems they have no real theme engine, just a method to replace background image for header and footer. duh.
Given how lofty office software generally is (especially when working on a big/dense document), I'd rather not have theme engine code dragging down performance for no reason.
Whilst you might want a single clean look, you definition and someone elses might differ.