Preview toolbar looks like this with Reduce Transparency ON:
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.... though, interestingly Preview's Settings has an option to set the background colour for the window, which affects the toolbar. I rather think that the buttons look quite nice with a stronger background, regardless of whether Transparency is reduced or not:
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... though I'm not wild about the sidebar shape.
I'd love to have that option for Finder windows, and other apps generally. Some colours work quite well:
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