I have to get around to writing this up properly, but basically, you just copy the resources/images folder out of your calibre installation directory and replace whatever icons you want in it. Use the sizes of the existing icons as a guide.
If you want to see the icons in your local calibre, follow the instructions here:
http://manual.calibre-ebook.com/cust...ates-et-cetera
If you want to package it up as a theme for others to use, go to preferences->miscellaneous->Create icon theme and select the folder
this will create the theme zip file
then make a post in this forum with the zip file and I will take care of making it available.