Color Theme still 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, select '---' and save it to set again the initial value

EXPECTED RESULTS

The initial value '---' can be selected and saved 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

Although selected, the '---' value is not saved, the same theme (chosen by user) remains applied, therefore the page's theme cannot be set automatically by the wiki level setting.

Also, the '---' value cannot be saved in Wiki Administration either. When selecting it and saving, the current theme still remains selected and applied.