How is the color of the “new messages circle” decided?

[chapoi]

@eecs_guy

What’s the class name for it?

This is easily found when you look at the DOM inspector. You can use this to override it:
for channels::
.chat-enabled .sidebar-section-link-suffix.icon.unread svg

for: personal chats
.chat-enabled .sidebar-section-link-suffix.icon.urgent svg

@eecs_guy

But on the header, only the personal chat indicator has a number that is the sum of all unread personal chat messages. The channel indicator doesn’t have a number. How can we make it show up?

There is no default way to do this as far as I know. You might need to look into making a plugin for that.