As we have all found out, gtk3 themes are still a work in progress. So I don’t have a good answer, but a thought.
The most stable (gtk3) theme I have found is GreenLaguna, I have mainly used this theme for several months. So make a copy of it and place that copy in your Home folder under /.themes. Change the name so not to get confused with the original. This will allow you to use your theme gui to switch to it. From there you can change the css files (color settings) without hosing the original theme. GreenLaguna has many color settings and I have not investigate it, so up to you to play with it.
System icons can also be customized with the same method by placing a copy (change the name) in /.icons to edit.
You can also change the panel layout with Mate-Tweak. If you already have a custom panel, be sure to make a copy (Save Panel Layout) of it first.
Themes are located in /usr/share/themes and icons in /usr/share/icons. Do not use “sudo” when making a copy to your home directory.