In the last few builds (I’m currently at 3261), there seem to be some icons/images missing. See below - the one for Set List Grid and the one in plugin editor windows for Show Plugin Parameters.
The red and yellow image is a Cantabile placeholder if it’s lost an image?
This is what happens when you use a custom theme, and a Cantabile update has more user-interface elements, that aren’t covered by your theme.
It can be a bit fiddly, but basically you have to merge the changes in the factory theme with your own customisations. I believe Brad is working on some features for a future release that could make this a lot more straightforward.