If it’s just the background, you’ll need to edit the background in a photo editing software such as Photoshop or Gimp and then upload it to your personal art catalogue, whether it’s an Episode background or a picture you chose from the internet. You can play around with the different settings (light, saturation, texture, etc.) until you achieve the look you want. There are many helpful tutorials on YouTube for this.
If you want more of a filter that applies to the characters and overlays as well, you have two options. You can use the @set hsl line of code. Here’s the official guide on how to use the command. You can preview how it’s gonna look on the preview and play around with the settings as well. Finally, you can also create your own filter on one of the editing apps I mentioned before, or download a premade one. There are also many tutorials on YouTube.
Good luck!