I thought I had my solution sussed, but no...
There are two Solarized themes, a light and a dark. They don't have an active/inactive title bar colour, so I was going to use the light theme, but with the title bar from the dark theme.
That worked, of course, but I found the theme slowed down my system. You read that right. A theme can slow down your system.
The search continues...
Tell you what, you sure can waste a lot of time with Desktop & window manager themes. Unless you can pick a theme (including icons) off-the-shelf, all the fiddling just eats up time.
I'm going to use Arc-Theme. It is quite old, but does provide theme elements for GTK2, GTK3 and xfwm4. It doesn't provide different colours for active/inactive, but again I can make my own by mixing/matching the light & dark variants of the theme.
I mentioned earlier that the use of some themes results in a slight system slowdown. Not a huge difference, just a hesitation that becomes annoying. My limited testing suggests the hesitation occurs with themes which use a lot of .xpm files. This is ironic as XPM is a format specifically designed for the X Window system. They are small text files which describe the colour to be used, a bit like a CSS declaration block.
The attached screenshot shows an example XPM file.