To change a Board Background, you first need to navigate to the "View boards" button in the bottom left-hand corner of your Space.
From there, hover over the board for which you want to change the background, and click on the 3 dots.
When you've clicked the 3 dots, another menu should appear that looks like the one below. Click on the "Change background" button from here to access the other Board Background options.
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After clicking this button, another pop-up will appear. You can choose to change your Board Background to a pattern, or a color. Once you've picked a new background, you can choose to set the background as a default for all new boards in the Space, and then click the blue "Confirm" button in the bottom right-hand corner of the pop-up.
Then, your Board Background will be changed to whatever you've selected!