Poweramp Skin SDK published here: https://github.com/maxmpz/powerampapi/tree/master/poweramp_skin_sdk
SDK includes:
Poweramp Sample Skin - complete skin, with two skin variants and extra options. Demonstrates various Poweramp v3 theming features. Skin can be built in most recent Android Studio
Poweramp v3 Reference Resources - default Poweramp skin layouts / styles / drawables / etc.
See https://github.com/maxmpz/powerampapi/blob/master/poweramp_skin_sdk/poweramp_skin_sam