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

Material themes are included for Lollipop ROM's only

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!

Screenshots

Settings in Light



Settings in Light with Dark actionbar



Messaging in Dark

For more screenshots see the support thread