org-fontify-done-headline

org-fontify-done-headline is a variable defined in `org.el'.
Its value is

nil

Documentation:
Non-nil means change the face of a headline if it is marked DONE.
Normally, only the TODO/DONE keyword indicates the state of a headline.
When this is non-nil, the headline after the keyword is set to the
`org-headline-done' as an additional indication.

You can customize this variable.