NEW UPDATE: AH HA!
I see in the addon details above that it IS INDEED noted that this only works for Windows and not Android. My bad, I missed seeing that but I got it now, all understood! I've set the star rating back to 5 because even though it doesn't work for Android, it still works awesome on Windows/PC's and looks fantastic! Give it a try! It just may become your top GO TO theme addon for Mozilla Firefox based browsers!

OLD UPDATE: This add on theme refuses to download, it keeps saying it's corrupt and won't initiate the download.

If it helps, here's some information about where & how I'm downloading:

Platform: Android

URL: https://addons.vaultpit.io/en-CA/android/addon/matte-black-v2/?src=search

My Useragent String: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.131 Safari/537.36

Thanks for your help!