Look in C:\Windows\Resources\Themes\ for a WP7 Dark folder and open up the wp7 dark.msstyles or wp7 mango.msstyles the one you want to use with a resource hacker open the image folder and find the images and edit them.
Image #508 6 states to this image
Images 863, 867
Note: You cannot edit a theme if you are using it. Switch to a different theme logoff/logon then edit the theme. Test, repeat