At least I think this is a feature request, I didn't see a way. If one were greedy, they'd ask for the same window customization tool as for palettes. I'd be happy with an attribute in the html body such as data-kmNSWindowIsOpaque, or data-kmNSWindowBackgroundColor, etc. which could be passed through to the OS when creating the window.
I'm curious. What do you see as the benefit from a window being translucent?
I know the macOS provides for this, but from my perspective it is only a distraction that makes the information in the window harder to see and quickly read.
a reasonable question. It's the exact same rationale for making palettes non-opaque.
IF one accepts the "expanded use" 
of the custom HTML prompt as something that can hang around and be a floating stats window, then i believe it helps having it take less visual command of the screen, and provide clues to whats happening beneath. to be fair, I'd experiment with the option and see if it has benefit. I may even mock something up.
As my stats project has gained features to report various vital info of my world, such as network traffic, pixel height/width of the current selection, and power output of my solar array / power draw of the coffee maker ... It occurred to me that translucency might be cool.
Not joking. Home power-stats live updated to KM Custom floating HTML. Y'mean that's not what it was originally meant for?...
using html collapsing "details" sections with dynamic window resizing also helps keep it less obtrusive, yet instantly available.
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