Is it possible to make an app that downloads all the settings from a server db and "makes up itself" ?
I mean to download some color codes (say, 3 or 4 values) and setup the views background color, text color, active text color, title text color.... as AppCompat uses in the manifest.
Yes, it is possible. There is also no need to use the AppCompat controls. Use the native controls. Android Material is the default theme on Android 5+.