Messages is a rewrite and the old iMessages is gone. It is meant to mimic the iPhone / iPad Messages. There is no option to change the message appearance like the old iMessages.
I've use the Pin feature to pin individuals and group conversations and that makes it easier to ensure you have the right message selected to avoid messaging the wrong people. I also have found, double-clicking an individual conversation or group conversation will open a new window which also makes it far less likely to message the wrong group or individual.
The highlight color is based on System Preferences -> General -> Accent Color pick a color that jumps out at you depending on Light / Dark mode and your personal preferences. This will show which conversation / individual is selected.