probably several ways you could do this.
Quickest I can think of is to alter the loop-single reply template
You should really do this in a child theme, as otherwise theme upgrades will overwrite this – google wordpress child theme video for lots of useful videos on how to do this – it’s quick and simple.
Create a bbPress folder in your theme :
Wp-content/%yourthemename%/bbpress
Where %yourthemename% is the name of your theme.
Then go into
Wp-content/plugins/templates/default/bbpress/loop-single-reply-php
Copy this file into the bbPress folder in your theme. bbPress will now use this file instead of the default, and you can amend this file.
line 61 does the content styling, so quickest to alter the background of this file.
so you would put a check I their for keymaster, and if so, use a different class that has a different background.
How up on coding and css are you, so that I don’t spend hours typing what you already know?
go to user>all users search for all your sites keymasters.
then click edit by each users avatar
after that you are in the edit profile page for that particular user and in the url it says
user_id=(number)
get all the user id numbers and then you can stack them using commas in a css code like this
.type-reply.user-id-1,.type-reply.user-id-2,.type-reply.user-id-3 { background: yellow; }