Docs for creating themes?

Hello,

is there some documentation about creating new themes?

I did not found a definition for the content structure of the theme zip file.

I can download a theme from the portal and modify it?

Can files be changed, deleted or can the image file names be chenged?

Are the images in the theme acessed from already installed portal components?

What are the directories ma, wa, scrindp and webgui for?

Greetings, Bernd.