Blackbox is a fast and minimal window manager for Unix-like OSs. A whole range of *box WMs started as Blackbox forks (e.g. Fluxbox, Openbox, Hackedbox). It comes with a set of styles which all have their own very tasteful and slightly muted background colors.
Preview#708090
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Workspace 1
12:11 PM
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Fog Gray
#aaaaaa
Dusty Blue
#7790aa
Deep Slate
#3a404b
Drab Olive
#444438
Fern Green
#77aa77
Storm Gray
#708090
Soot Gray
#333333
Heather Purple
#9077aa
Clay Red
#aa5555
Basalt
#444444
The background colors are defined in the styles bundled with the source code. The background color is defined as *backgroundColor. I used the source code mirror at GitHub.