Firefox Color - Corrupt File

Hey peeps … I’m new to this so please forgive my NEWBness. I created a Firefox Color theme but I can’t seem to find a way to set it to default. I’ve saved the file to my PC and tried loading it into themes but it says the file is corrupt. I’ve created a Firefox account and uploaded the file to the Themes area but when I try to install it that way it gives me the ‘file corrupt’ error again.

Am I missing something here? Shouldn’t I be able to save a Firefox Color scheme that I set up and use it as the default for the browser? Browser is up to date and using Windows 10 64-bit. Any suggestions you could offer would be appreciated. Thanks.