So far, so good! I'm using the Mercury theme, and wanted an enabled effect to be more obvious in the FX rack. Using the Theme Editor and an image editor, I was able to lightly outline the effect's button in green to indicate it's enabled.
A couple of tips:
This required installing the Theme Editor and 7.2016 Platinum early Access build.
If the Theme Editor crashes on saving, check to make sure the theme being saved isn't currently active in Sonar. I think this is mentioned in the documentation or in a post.
Next, I want to try Craig's Punch-in button mod. Like others, I've inadvertently left punch-in enabled when I wanted to record an entire take. That can be frustrating!
Modifying just a few theme elements can take time. It makes one appreciate the effort by the developers to create a whole new theme.