I am not familiar with The Backrooms, but I tried my best.
But because this Mod is so customisable, you could make an Add-on Mod with your dimensions, or tell me what you want, I will gladly add them.
This is a very small Mod, it only adds:
-Five basic dimensions.
-One block.
-One biome.
It was made for CactusPlays when they asked for something Backrooms themed.
To enter the "Backrooms" you need to craft Limenal Blocks.
This will give you the recipes for each Portal Igniter.
Simply make a portal out of Limenal Blocks, and use the igniter of your choice.
I will not tell you about each dimension so that you can find out for yourself. Sorry.
