Is your feature request related to a problem? Please describe.
With the default Blizzard interface (when ElvUI is not activated), the GSE windows is very transparent. Some players might not be annoyed by that, but others might be.

Describe the solution you'd like
It might worth considering to add an option to let the user (the player) set the alpha-transparency for the background of GSE windows.

In the Options, maybe you could rename the "Colors" category to "Aesthetics" (or something like that).
Then maybe you could add this new "Background Alpha-Transparency" setting there, in "Aesthetics".

Describe alternatives you've considered
Make them fully plain color (not half transparent). But I kind of like the window being slightly half transparent.

Screenshot

This was a change with WoW Shadowlands in that the default background stuff changed and this was the outcome. I could potentially look at adding function to turn on and off a background texture

Not going to do this. UI Mods like Elv and AddonSkins provide this functionality.