Custom theme, built on a different machine. I thought I knew how to install themes, but apparently not. Somewhere along the line I had written down a different path for installing core3_theme files, and they almost work right when you install them there... for everything except Crestron App projects, apparently. Installing the core3_theme file in the correct c:\ProgramData\Crestron\Core3\UserThemes folder allowed it to work correctly.
Thanks for reminding me to check my assumptions...