Hi there! Thanks for the purchase ^^
If I understood what do you mean by background, you will need another plugin of mine that let you add a background to any scene or window. You can add parallax background, sprites, and static images(Paid):
https://hakuenstudio.itch.io/hakuen-studio-background-manager-for-rpg-maker-mv-m...
But if you mean, change the windowskin, then you have to use this one(Free):
https://hakuenstudio.itch.io/eli-change-window-skin-for-rpg-maker