From what I understand in today's perspective, this idea did not go anywhere because the limitation of the current UI is quite easy to go around: If the half screen preview is too small, then you can either:
Open the colortheme page in viewmode to get a nearly full screen preview
Open a regular page with the colortheme applied on it.
Zoom out
Larger screen resolutions are more common, with enough screen width, the user stays on desktop mode styles even when the preview is half the screen wide.
On the concept itself, transparency should be limited to what's necessary. I don't have sources backing up my claims, but it's never good to partially hide part of the UI for accessibility: https://www.w3.org/WAI/WCAG22/Understanding/target-size-minimum Links and buttons are target inputs and should not be completely covered by another element.
All in all, with today's knowledge, I doubt we'll ever want to implement this idea.
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