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Is there a way to strech thems on the iphone 5 so that it takes up the whole screen?

Themes aren't run like apps. Themes will pretty much replace system images. If it isn't optimized for the iPhone 5 it will probably just leave things unthemed.

There are cydia tweaks like force full I believe that stretch non iPhone 5 apps to fullscreen.
 
I have Full Force but it's only for apps, just did know about the themes. Found some really cool ones but they dont take up the whole screen.
 
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