Ref: https://community.bistudio.com/wiki/systemChat

If easier, just for https://community.bistudio.com/wiki/customChat should be a good start.

Would allow different font, size, color, images, etc.
Sample in complete chat replacement mod: https://forums.bohemia.net/forums/topic/222073-extended-chat/

Would allow to re-create the standard chat channels in mission intro and outros too (only systemChat works. See: https://forums.bohemia.net/forums/topic/174511-radio-conversation-in-intro-outro-section-how-to/)