It has the Firefox logo in the middle, and quick links on the bottom for things like history, bookmarks, settings . . . . . . .

Thanks for your response. This does 'nt have a Firefox Logo in the middle. It appears on the Appearance page of add-ons. It is a white figure on a black background. It simply is Identified as the default theme, and in additional information, it is identified as Default 50.1.0 by Mozilla. I have tried to remove it, it does not show the option of activating it or removing it.

Thanks for your response. This does 'nt have a Firefox Logo in the middle. It appears on the Appearance page of add-ons. It is a white figure on a black background. It simply is Identified as the default theme, and in additional information, it is identified as Default 50.1.0 by Mozilla. I have tried to remove it, it does not show the option of activating it or removing it.

The "Default Theme" is as name says the default complete Theme that Firefox comes with.

If you want another theme then see https://addons.mozilla.org/firefox/complete-themes/ and these are image backgrounds https://addons.mozilla.org/firefox/themes/

THis is the default theme this is built-in each Firefox release and used when you do not use a custom theme.

Thanks for your help, it was just something I had never seen before.