OK, I stumbled upon this solution for Applying a Persona(lightweight)
Theme to any Third Party Theme regardless if the <em:skinnable>true</
em:skinnable> is present or not and without using any extension.
Works with Firefox 4 RC1 and Latest Nightly build 4.0b13pre:
1. First go to Add-ons Manager and enable and Apply any Persona
Theme(Restart of
browser may be required)
2. Then after applying a Persona Theme go back to Add-ons Manager and
enable any
Third Party Theme of your choosing.(Restart of browser is required)
3. Now after applying the Third Party Theme, go to urlbar and type
about:config and toggle pref: lightweightThemes.isThemeSelected : to
true.(Now Restart browser)
After the restart you'll see the Persona Theme and The Third Party
Theme applied together.(Additional Restarting of Browser has no effect
on your newly combined Theme.) Repeat steps above to try other Persona
and Third Party Theme Combinations.