The

macro doesn't seem to allow any sort of highlighting in the code. I don't mean using different colors for the text based on the code language (that's not highlighting, by the way), but putting a yellow background behind some lines, or just making them bold.

If you see a block of code and two lines have yellow backgrounds, your eye is drawn right to those lines. It's a well-understood method of emphasizing something. It's quite disappointing to find out that Confluence not only can't do that but blocks all attempts by the user to do that.