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When i need to make new themes i get the specs from existing iphone themes and create new images inside photoshop saved as .png set to those dimensions.

With a .strings file editor you can change the text settings and content.

Anything more complex is above my pay-grade.
 
Make a folder, and name it XXXX.theme

Make another folder in that called icons.

Place .png files in there, name them the same as the app you want to theme.

SSH it to your iPhone.

Activate in Winterboard.
 
I am very (very) new to this, but I've found the best way is to examine other themes. Look through all the folders. See what goes where. Change bits and see what happens.

If you know HTML, JS & CSS then that's a great help.
 
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