gnus-header-face-alist

gnus-header-face-alist is a variable defined in `gnus-art.el'.
Its value is

(("From" nil gnus-header-from)
 ("Subject" nil gnus-header-subject)
 ("Newsgroups:.*," nil gnus-header-newsgroups)
 ("" gnus-header-name gnus-header-content))

Documentation:
*Controls highlighting of article headers.

An alist of the form (HEADER NAME CONTENT).

HEADER is a regular expression which should match the name of a
header and NAME and CONTENT are either face names or nil.

The name of each header field will be displayed using the face
specified by the first element in the list where HEADER matches
the header name and NAME is non-nil. Similarly, the content will
be displayed by the first non-nil matching CONTENT face.

You can customize this variable.