Click the Design tab in the upper blue bar:
From here, you can:
CHANGE YOUR THEME
CHANGE BACKGROUND (depending on your template)
The most important thing to know about the Design tab is that Weebly will always open into your personal template. If you accidentally click Design and change your theme, don’t worry. You can simply click Design, Change Theme, Custom Themes, and click on your custom theme name (listed on your Personal Reference Sheet) and your theme will return exactly as it was.
I don't teach this tab because the design of the site is completed by the time site updates are being made. You'll be using the Build, Pages, and Settings tabs to make updates. If you need to know about the Design features, you can click the ? icon for the Support Center or check out the links below: