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
Material Light
Material Light with Dark Actionbar

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

Step 1. Download Holo Themer Premium Mod (Paid for free Free purchase) APK file on HappyMod.com.
Step 2. Once it's downloaded, open Downloads, tap on the APK file, and tap Yes when prompted.
Step 3.Holo Themer Premium Mod (Paid for free Free purchase) APK will begin installing on your device. Simple.

Step 1. Download HappyMod APK file on HappyMod.com.
Step 2. Once it's downloaded, open Downloads, tap on the APK file, and tap Yes when prompted.
Step 3.Search Holo Themer Premium Mod, click Download.
Step 4: You can download 100% working mods on HappyMod with one click.