You can create a new design or update an older design to sport a gradient color background.
To update your design background, click 'Background' at the bottom of the left menu. From the expanded options, select 'Gradient'.
Gradient background comes in 2 forms:
Click the 2 color palettes to select your color from the color wheel. Use the preview to test, mix and match colors. Once you're satisfied with the results. click 'Apply' to save your background.