you need to zip it at root leve l and use NO compression (aka. “store” in most zip programs)
(root level: highlight all files/folders right click and zip. if you extract the zip and end up with 2 folders … then you did it wrong
eg. (extracting the zip and checking that you zipped it at root level)
MyTheme > MyTheme > images and xmls X Incorrectly Zipped
MyTheme > images and xmls 
Correctly Zipped
Also note: 30MB is the Maximum Size for a Theme
