This might help.
Hey I have easy idea for you,
You can simply filter the background in the portal by using Hsl tools or you can use this code.
@set hsl 0 0 -76 no_colorize with blendMode Normal to 100%
You can find this option in the portal under the previwer area in “Show Helpers option” under this, in" More Helpers" Voila you will get Hsl filter then Turn it on it and Select Lightness and Drag your mouse ponter into your left side or swipe to your left side you will notice that background colour chages into Darkness.
Hope this helps:
I have a version created for night with bright city lights if you still need/want it:
PM me if you want the big version :)
thank you smmm