Start with Apk Manager to decompile the app. From there check out the AndroidManifest.xml first. Search for Theme.Light and change to Theme.Black. After that search in the values folder in colors.xml and styles.xml. Then you nay have to search all the xmls in the layout folder for static values. There is also images you may have to change as well. Last would be smali but usually is just a couple of things.