@headings-color doesn't revert to #000000 (black) when rolling back the Color Theme

Steps to reproduce:

1. Go to Administration: Presentation and click on Customize for the current color theme (http://localhost:8080/xwiki/bin/edit/FlamingoThemes/Charcoal)
2. Click on "Base colors"
3. Pick a color for the @headings-color property; for ex. use #ff0000 (red)
4. Save & View
5. Hit Ctrl + F5 -> the red headings color is properly displayed
6. Go to http://localhost:8080/xwiki/bin/view/FlamingoThemes/Charcoal#History and rollback the document to version 1.1
7. Again hit Ctrl + F5 or edit the theme and check the @headings-color property

The headings color remains red instead of reverting to black like it was in 1.1.