Hello there!
Here are the customization options available to anyone:

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1. Color of the gradient on the realm join screen.

2. The realm title logo on the realm join screen.

3. The in-game logo with your own custom realm join logo.

4. The custom painting only admins can place.

5. All the dimension background images.

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If you want this done, you can contact "maragi.developments" on Discord to help you set this up and customize your addon.