There is a user.js projects on github (https://github.com/pyllyukko/user.js/, https://github.com/ghacksuserjs/ghacks-user.js/). But we can not use them if we don’t have rooted device. because we can not inject these scripts.
But if you use non-rooted device and use Firefox, you can inject the scripts by usb remote debugging the about:config page. here is the simple explanation:
But for remote debugging we need mozilla’s firefox. The mozilla firefox desktop edition does not recognize other browsers
I know there are license problems with Mozilla. So what do you prefer? Is there any solution do you know?