The variable *editor-pane-composition-selected-range-face-plist*
is a plist that is used to modify the face of the composition string when :selected-range
and :selection-needs-face
are passed in the plist to editor-pane-default-composition-callback. The plist is merged into the plist that is passed into editor-pane-default-composition-callback, so keywords in it override the keywords in the face.
CAPI User Guide and Reference Manual (Macintosh version) - 3 Aug 2017