Hi,
the “regular” message (not a direct message, but unread messages in a channel) is the background: var(--tertiary-med-or-tertiary); colour. So this is derived of whatever you have set as your tertiary accent colour.
By default this is blue-ish
If you don’t want to change your tertiary colour, you could override this one specifically by setting a different background for .chat-channel-unread-indicator in your CSS.
New direct messages are green by default, which is the var(--success)
Hope this helps!
Cheers 