different theme output for desktop and mobile

Try using Chrome Dev tools – it has device screen size simulators built-in and many other useful tools https://developers.google.com/web/tools/chrome-devtools/

And to see any changes try clearing the browser cache:

  • Open Dev Tools by pressing: F12 or Ctrl+Shift+I (or on Mac:
    Opt+Cmd+I)
  • Now by just leaving dev tools open, right-click or click and hold the
    reload button next to the address bar. Now a somewhat ‘hidden menu’
    opens.
  • Choose: “Empty Cache and Hard Reload”