ediff-use-faces

ediff-use-faces is a variable defined in `ediff-init.el'.
Its value is

t

• Automatically becomes permanently buffer-local when set.

Documentation:
If t, differences are highlighted using faces, if device supports faces.
If nil, differences are highlighted using ASCII flags, ediff-before-flag
and ediff-after-flag. On a non-window system, differences are always
highlighted using ASCII flags.

You can customize this variable.