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

The premium version lets you make your own themes

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!

Storage permissions are for backups
Internet permissions are for ads and addons (no ads in premium)