Due to the CSS the color palette totally changes in 12 not sure what you can do other then try and create custom colors. As you saw total apply a theme even the default no theme unless you want to play havoc with your UI. Just one of many reasons I am holding off on 12 for a while. Good luck.
I had the fonts turn white. The cause was the fonts were not enabled on my Mac. After enableing these fonts the colors appeared to be fine.