Page's Color Theme cannot be reset to initial (default) value

STEPS TO REPRODUCE

1. Create a page (e.g. Pag1)
2. Go to Administer Page > Look & Feel > Themes
3. Change the Color Theme from '---' to another one from the drop-down
4. Click 'Save'
5. Click again on Color Theme drop-down and select '---' to set again the initial value

EXPECTED RESULTS

The initial value '---' can be re-selected in order to keep the original setting so that when the wiki level color theme is changed, the change to be also applied automatically to the current page (like Icon Theme).

ACTUAL RESULTS

The value '---' is not displayed anymore after the color theme is manually changed at the page level, therefore the page's theme cannot be set automatically by the wiki level setting. As a result, the pages to which that have been set a different theme than the one set at wiki level have to be manually updated (if the user intends that these pages to have again a matching color theme set).