What controls sidebar color and certain sheets' color?

[samdale67]

What setting, if any, controls the sidebar color and certain sheets. I'm talking in particular about the sidebar in Finder and Mail windows. Wherever this color comes from, and I have not been able to find a setting to change it, does not work very well with my highlight color, graphite. See attached screenshots—I have color marked with red circles. What I think I want is the option to control this color or to turn color completely off. By the way, I have "reduce transparency" turned on in Accessibility. I'm using the light theme.
[doublepost=1533076306][/doublepost]Upon further testing, I can see that the sidebar color reacts to colors used in the wallpaper.

 

[samdale67]

What is it with the sidebar! It's weird, and it's getting weirder!

 

[Binarymix]

A lot of UI elements are partially translucent with a Gaussian blur added, so the colours react based on the wallpaper and other windows/elements behind the active window.

You can see this especially in safari where the window chrome becomes coloured based on the webpage loaded. I find that particular use case kind of annoying.