Did you get them from here: https://addons.mozilla.org/en-US/firefox/themes/ ?

Yes, I did. I had Little Fox running and switched to default to look at it. Now I can't get back, but it did work once.

They'll be listed in Tools | Add-ons | Themes I think. You can enable/disable them from there.

I am looking under Add ons|Appearance. The themes that I have listed all show there. Each of them has a button to remove, but there is no button to select it or turn it on.

Maybe the reason the Enable button is missing is because that particular theme is no longer compatible with Firefox 4.

Can you try another one just to test it? https://addons.mozilla.org/en-US/firefox/themes/

I had just downloaded FF3 for FF4 this morning, but I deleted it and downloaded it again. It still has no enable button. I also downloaded MX3. No enable button. Also, remember that I had the LittleFox running until I foolishly changed it just this morning, so it must be compatible. Meantime, not of them have enable buttons!?!

I downloaded FF4 again. It seems to have fixed the problem after a fashion. I think that anything that I download since the reinstall is showing an enable button. I guess we can say that my problem is solved. Thanks for your help.

You're welcome.