Customizing Ubuntu 18.04 UI - How to switch colors for certain UI items?

I have the default looks, at least brought forward from around Ubuntu 16.10. Now I have Ubuntu 18.04 and have a few little discomforts:

1. It's hard to tell which window is active - light gray vs. less light gray window name is clearly not enough to distinguish quickly.

2. Some programs paint black items in selection boxes, which are almost black in Ubuntu's Ambiance. For instance, Inkscape's selection of stroke style. And in general, I want to invert the selection box everywhere.

3. Tabs - again, it's hard to see which one is active. I'd like the active to be much more constrasted.

I have looked at few other questions like this but the answers either apply to Unity, or are obsolete...

$ sudo apt-get install gtk-theme-config
E: Unable to locate package gtk-theme-config 

and I am a bit afraid what my experimentation would do on 18.04. In the past, when I was trying to do something with themes, I most often it screwed up and couldn't revert.

What is the way of changing colors of UI elements in Ubuntu 18.04?

PS: Switching to Radiance only helps with 2).