A theme is a combination of a background and default color set, fonts, icons, and other design elements.
To change your project's theme:
1. Deselect any objects in your project or click outside of the canvas.
2. Open the Settings tab in the right sidebar, then click on the thumbnail to toggle to a different theme. This will change chart and text field colors, the font type, and in most cases - also the background color.
Note: the charts and objects visible in the thumbnails are just illustrations for chart and background color combinations, you can create any type of chart within any theme.
3. To customize the background color, click the colored square below the theme selector, under Project background. You can do that even after choosing the theme.