Okay. Digged a little deeper. The filter Malware Domain interferes with the image at the add on bar. If it is disabled, the image is displayed.Instructions to reproduce (Fresh profile):
1. Install personas. Restart.
2. Install adblock plus. Restart.
3. Enable Malware Block option after restart. [Theme still appears].
4. Restart. [Add-on bar theme disappears]
Follow-up to the issue (Maybe unrelated):
5. Open a new window. The add-on bar displays the theme.
6. Try disabling the filter and restart. [Theme appears]