Hi @Matt The Banana ,
Welcome to the group!
If you want any other background that is available in Trello free version choose among them but if you really want to apply black color then visit https://help.trello.com/article/818-changing-board-backgrounds and read the document carefully.
OR here is some of the information
Click the + to upload a custom board background.
You can also hover over your uploaded image and click options to set how the image displays and to choose either light or dark board text depending on the color of your image.
Choose how your custom board background displays.
Since Trello in the browser is dynamic and the background image adjusts to the size of your browser window, there is no ideal pixel resolution for a custom board background. We recommend using a high-quality image that has been taken in landscape mode (wider than it is tall) for best results.
Hi, Atlassian Community! My name is Brittany Joiner and I am a Trello enthusiast and Atlassian Community Leader. I'll be sharing with you a series of videos with my top tips on how to best use Tre...
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