There is probably something different about the DOM at mobile which stops your style applying, or there is an existing style which is taking precedence and overriding yours.

You should be able to debug the course on a desktop by shrinking the window to mobile size.

Or you can debug a phone: If it's a recent Android phone it's quite easy to debug, with Chrome on the phone and Chrome on the computer and a usb cable. If it's an Apple phone you'll need a mac with iOS and Safari on the phone and Safari on the computer and also the usb cable.