I have been struggling with this problem for a few days now and I am running out of hair to pull out.
I do a basic install on my web server of any ss3 version and it installs fine and runs fine with the 'simple' theme but as soon as I upload a new theme flush the system and select the new theme in the admin area I get back what looks like a debug theme screen with a blue bar across the top but all content displayed with out formatting.
I have checked the file permissions and they are the same on 'simple' as well as my theme which is derived from 'simple' and modified.
I have run dev/build?flush=all with no change
I have also tried renaming the theme but that didnt help either, I am using 'scr' as my theme name.
I have tried setting the environment to 'dev' and i get no additional info.
PLease help as I have run out of ideas.