To modify the background:
1. First, click to select a block.
2. Next, click Background in the left side panel.
3. Then, select a pattern or texture under "Background Image". You can also change the Background Color (found above the patterns / textures) and decide on a color that complements your content!
To change the color, click the background color paint tool icon, and choose a default color or enter a hex code. To add a new color, click the + sign under "Recent Colors" to choose from a color slider and color wheel. The color chosen will preview in a box to the left of the hex code, and you can click anywhere outside the color picker tool to set the color!
You can also add your own image as a background. Have experimenting with your backgrounds! :)