Holo Themer lets you force any app to use:
Holo Dark
Holo Light
Holo Light with dark Actionbar
Holo Dark with no Actionbar
Holo Light with no Actionbar
Wallpaper
Wallpaper with no Titlebar
Pure black
Pure black - no actionbar
Device Default
Device Default - Light
This is the premium version and allows you to also make your own themes (choose background colour, text colour etc)
This will only work on apps that already use a Holo theme of some kind (which is most apps, and practically no games)
Even then, some apps will mess up a bit
The new Add-on store lets you download additional xposed mods which work with this one
EG: the Settings_light addon makes the icons down the left of the settings app black, to use with Holo Light
Visit XDA thread for more details
Credits to @NeverUseful for the icon!
IMPORTANT
The mod requires Xposed Framework
It is highly recommended that you try the free version before buying premium