You can Download the official patch from the publisher's website. Just follow the link, Confirm you are 18+, and register an account. Then click the "Download now" button and choose the windows installer.

https://www.johren.games/games/download/dokyusei-remake-dlc-en-zh/

1. Make sure you have the game already installed on Steam
2. Unzip the patch
3. Copy the contents inside the "update" folder and paste them into the root of your game's folder, when it prompts you to overwrite a file click yes

Easiest way to access the root of your game's folder is to right-click the game in your library > Manage > Browse local files

If the patch was installed correctly your game's main menu should now have a nice background instead of being blank.

Before...

After...