When viewing the main forum you see this:

How can I change those textballoons which show if there are unread threads with other icons/images?
I've looked in the "Icons & Logo's" and in the "Manage resources" in the Theme settings, but could not find them. Theme is a copy of the original theme.
 

Phuu... found it. Have to change it forum by forum. I thought it could be done all at once but this has more options to it. Nice!

 

Now I changed the icons, but if there are unread posts, it's hard to see.

Is there a code which I can use in custom.css to have them give a red background color if there are unread posts in a forum?

 

to change background color then in custom.css try:

.ipsItemStatus.ipsItemStatus_large {
  background: red;
}

 

@MariusThank you, I knew about those, but would like to use my own custom icons which can be put in at the forum settings with icon.

@steve00Thank you. I'm having a thread in theme customisation for a kindlike code for unread messages, but that only works on default icons.