Congratulations, you have now customized the home button in your theme.
Customizing on different platforms - You can repeat the steps, starting from Step 3, to edit the Lollipop or Marshmallow image as well. Customizing multiple platforms with different images lets you make themes that look appropriate for their target platform. The design of Lollipop and later are often quite different from KitKat, and in most cases you will probably want your theme to reflect that. However, if only a KitKat version of a resource is set, Theme Creator will automatically try to convert it to work for other platforms as well.
After you have created your theme, if you want to learn about alternate ways to edit images, or about interactive previews, please see the appropriate sub-sections.