Check out this blog post by Bok on how you can create theme friendly buttons in WP7. He outlines how you can make sure your icon buttons are still visible after users have customized their phone color themes. This is also particularly important for the Marketplace certification process where these conditions are tested, to determine whether your app and its UI elements are still visible after user customizations. Here’s a snippet of Bok’s post. I’ll let you click to his entire post. Also check out his app up on Marketplace – “Currency 7”
http://wp7dotnet.blogspot.com/2010/11/create-theme-friendly-buttons-in-wp7.html