When you apply a new theme to a webpage, iWeb automatically converts the content on the page to the new theme’s colors, fonts, and layout.
Select the page you want to change by clicking it in the sidebar on the left of the iWeb window.
In the toolbar at the bottom of the iWeb window, choose a new theme from the Theme pop-up menu.
When you change themes, content might change position on the page.
To revert to the previous theme, choose Edit > Undo Change Theme.