Comment 13 for bug 1210568

Thank you, that helped. The clue was 'Default': '' in system-theme-associations, which shouldn't have been empty.

What might have happened is that onboard-defaults.conf (system defaults) didn't exist the very first time you started Onboard. This wouldn't usually happen when it was installed via package manager, but might easily if you run from source.

Trunk has a likely fix now. The system default theme is always considered, not only on first start. This only stops once the theme was manually changed in preferences.