only basic "picture" themes will display.
I've tried multiple themes on two different devices, but the only themes that will display are ones that change the background image and nothing else.
- User Agent: Mozilla/5.0 (Android 5.1.1; Tablet; rv:68.0) Gecko/68.0 Firefox/68.0
At the present time, only themes that have an image (as opposed to those that are colours) will display correctly in Firefox for Android. This is an issue that our developers are aware of and which may be fixed in a future update.