I really like the Yaru dark themes because of the nice icons and the soothing dark colors. But I have a problem when there are multiple windows open, since their borders are not very distinct (at least for me).
When I played with the high-contrast-inverse theme, the thin, colored border was a big help. (However, I don't want to use that theme.)
How hard would it be to tweak the Yaru dark themes? Or does someone have a suggestion of a modern, Yaru-like theme that has colored window borders?
Second that motion. Particularly for me is the difficulty of noting which window is currently active, when the "titlebar" is always black.
Common suggestions are to alter the gtk.css in the titlebar or headerbar properties, but whatever is at the top of Yaru windows doesn't seem to be either of those elements. I looked at it with GTK inspector, and can't find any css element corresponding to the titlebar.
My temporary workaround is to color the menubar and toolbar elements in a dark green background, matching the mate theme, which helps.
But I would love for the MATE version of Yaru, at least, to have a more visible difference on the titlebar.
Speaking of Yaru theme tweaks, I'd like to know if anyone has tips on how to create a dark panel for the light theme. Basically, the color scheme Mint-Y uses.
I finally found a solution that was already close at hand.
After poking around a bit more, the solution (for me at least) was to open Control Center, then click Appearance/Customize/Window Border, then select another Window Border. Both Atlanta and HighContrastInverse work very well for me, allowing all of the nice Yaru icons and style to come through, along with good contrast between windows.
So I'm happy now. 
Mine are all the same colour, however with the theme I've chosen, the window decorations are coloured and are not coloured on the inactive windows.
Thanks for the reply.
Your screen looks very nice, but there isn't enough contrast for my old eyes. The terminal window simply blends in too much for me, even with the colored window action buttons.