Theme problem

I am trying to apply a theme to both Firefox and Thunderbird.

TB>  https://addons.mozilla.org/firefox/1947/
FF>  https://addons.mozilla.org/firefox/72/

I follow the instructions:

Thunderbird> Save to desktop> Themes> Install> when I select it it show for a split second then diasappears and the theme remains.

It does this through Firefox too when I click the install button? Am I doing something wrong?

I used to have this theme but had to format and reinstall everything on my machine. I run XP SP2

Thanks

Adam