From the slide background tool (the blue photo icon on the left), you can choose to add images, charts and graphs, or solid colors. The tab for solid colors is to the right of the pie chart tab, and is indicated by a rectangle divided diagonally (half is dark and half is light).
You can choose from Haiku Deck colors unique to your theme, or pick your own custom solid colored background. To choose your own custom color from the solid colors tab, just look for the eyedropper button.
Use the outer ring to choose your color, and the square in the middle to choose how bright or dark it is.
If you've chosen a solid color as your slide background, you can switch to any other slide background by simply choosing a chart, graph, or image instead.
To see it in action, check out our tutorial video:
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