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 bucket icon, and choose a default color. Or, click the + sign under "Recent Colors" to choose from a color slider or paste a hex code. The color chosen will preview in a box to the right of the color slider.)
Have experimenting with your backgrounds! :)