Firefox places lightweight theme images as lightweighttheme-header and lightweighttheme-footer in the Firefox profile folder (xxxxxxxx.default). The images will be resized and copied to the lwtheme folder if you select the theme. You need to delete the files in this folder to make Firefox use a new lightweight theme if you replace the lightweighttheme-header and lightweighttheme-footer yourself. Set the lightweightThemes.usedThemes and lightweightThemes.selectedThemeID prefs to make Firefox use the lightweight theme images. You can change the values of background "accentcolor":"#" and "textcolor":"#" to make the text show properly and possibly change the name and author as well. .... You can modify the name and label property in lightweightThemes.usedThemes as you like. Set these prefs on the about:config page.

lightweightThemes.persisted.footerURL = true
lightweightThemes.persisted.headerURL = true

lightweightThemes.selectedThemeID = 1
lightweightThemes.usedThemes:
[{"id":"1","name":"Unknown","headerURL":"https://addons.cdn.mozilla.net/","footerURL":"https://addons.cdn.mozilla.net/","textcolor":"#","accentcolor":"#","iconURL":"","previewURL":"","author":"Unknown","updateURL":"","version":"1.0","updateDate":"","installDate":""}]

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• http://kb.mozillazine.org/about:config

I found the xxxxxx.default folder (appdata\roaming) and all of the files you talked about, thanks! One question, and this might sound stupid, but the two prefs you mentioned (lightweightThemes.usedThemes and lightweightThemes.selectedThemeID) are just found in about:config, not in the xxxxxx.default folder, right?

Yes, the prefs I mentioned can be found on the about:config page. Once you created them they will appear in prefx.js after you have closed Firefox.

The lightweightThemes.usedThemes pref is the pref that stores all (up to 50) themes that you can install. Only the images of the currently installed theme are stored in the profile folder. When you select another theme then Firefox will use the data stored in the pref to retrieve the images of the new theme. So you can install a theme and then backup the images and possibly its data from the lightweightThemes.usedThemes pref.