I've always thought the stock game Customizer Room aesthetics to be very odd with too much clutter in the background.

I found a mod to replace the room background called Room customizer by veryslow 5 colours   https://www.klubexile.com/files/file/4340-room-customizer-by-veryslow-5-colours/  

There are 5 files to choose from. I installed Custom.AAA.RoomCustomizerDefault which provides a plain lilac background. There's also red, pink, blue and green options.

This file goes in Addons folder and will be installed when the game is run. I have installed all of the Hook 5 files and mods with Generic Mod Enabler and will walk through the installation process with this utility. When Generic Mod Enabler is first run, a folder named MODS will be created in the game folder.

In the MODS folder , create a sub folder [Customizer Room] and a sub-folder named Addons and place Custom.AAA.RoomCustomizerDefault in there.

Here is the directory tree:

+---[Customizer Room]
¦   +---Addons
¦       +---Custom.AAA.RoomCustomizerDefault

Run Generic Mod Enabler (JSGME). On the left side of the program window is the list of available mods for installation and on the right side is the list of installed mods. Select [Customizer Room]  and click on the '>'  icon to install and enable this mod. [Customizer Room] will now appear in the Activated Mods list on the right side. The Customizer Room mod is now installed. It can also be uninstalled by selecting [Customizer Room]  and clicking on the '<'  icon.