Radio buttons are confusingly grayed out
The radio buttons in the property editor are grayed out(using adwaita theme on fedora 28). I can't speak for others but I personally found this confusing when trying to learn how to use it. I assumed that I was unable to click on the radio button in order to add custom .png for a gtkimage widget, it was only after exhausting other options that I came back to it and tried clicking on the radio button out of frustration. It's a minor usability issue, just thought you may be interested.
Thanks for taking the time to read this issue and for contributing towards the project.