Assigning a Style to a Group
A highlight style is assigned per group, in ACP under Users & Groups → Groups → (add/edit a group). Two fields appear on the group form, added by this extension:
Post highlight style
Which style (if any) this group's members may use. Set to "None" to disable highlighting for the group.
Highlight by default
If enabled, the highlight toggle is pre-checked whenever a member of this group starts a new post or reply. Members can still switch it off for that specific post.
Highlighting is tied strictly to a member's default group: the same group phpBB already uses to colour their username everywhere else on the board. If a member belongs to several groups, only their default group's style applies; being in some other group that has a style assigned has no effect unless that group is also their default. This keeps a highlighted post's appearance consistent with how the member is already presenting themselves board-wide.
If a member's default group has no style assigned, they won't see the highlight toggle at all when posting or replying; there's no separate permission to grant, group membership is the only gate. See The member highlight toggle.
Changing a group's style later
Because a post's highlight is resolved live from the poster's current default group every time it's rendered (never stored as a snapshot), reassigning a group to a different style (or to "None") immediately changes how every opted-in post from that group's members looks, without touching a single post.