UserStyles for XML activities? All my android phones so far have been Samsung's, with AMOLED screens. 
One of the benefits of AMOLED is that black saves battery life. 

So many apps though (including many big ones, even from Google) don't include a dark theme.
So you wind up with a lot of white, sucking down your battery. 

This got me to wondering, seeing as the Android UI is built on XML with something essentially like CSS, is there any way to override the styles in apps? 

I'm thinking something along the lines of the Stylish add-on for Firefox, which allows you to specify user stylesheets that can override all sites, or specific sites. 

I am rooted, and was thinking maybe something on that Xposed Framework?
(I've never used any Xposed modules). Or any way really. 

Thanks in advance.
Try Holo Themer which is an xposed module that will force all Holo apps (that is, apps which are consistent to the 3.0+ UI as opposed to the pre-Honeycomb UI) to be either dark or light.

If, after you install xposed, your phone won't boot, boot into recovery and flash the disabler attached to the thread.