Here's the wiki page about it if you want to read it in full:
http://claw.wikidot.com/custom-icons
Basically:
1) Create a new folder in the Flash UI directory called Themes.
2) Create a new TXT file in the Themes directory called themes.txt
3) Download an icon pack that you want to use. Extract the files of the icon pack into a folder (name it something short and easy to remember)
4) Put that folder into the Flash UI\Themes directory
5) put the name of the icon-theme-folder into the themes.txt file in the following format:
Example themes.txt file (don't include the ---, just what is between the dashes)
---------------
<Themes>
<Theme name="Mario_by 32x"/>
</Themes>
---------------
You can put multiple lines in there too. just make sure you put it like this:
<Theme name="(title of icon theme folder)"/>
hope that helps.