Note: in API>=21 to get the Actionbar transparent you need to get the Statusbar transparent too, otherwise will not respect your colour styles and will stay light-grey.
Values/styles.xml: you can use these themes in your AndroidManifest.xml to specify which activities will have a transparent or colored ActionBar: We start by using an appcompat-21 dependency.
I've finally found a solution and since it took a bit of my time I'll share it to save some of yours.