In order to switch to a different design theme, first install the desired theme in section "Design → Themes" in the design editor of any Webasyst app.
You can also install a design theme in the "Themes" section of the built-in Installer app.
Then open the design editor's "Settings" section of every app generating your website pages, select the new theme from the drop-down list, and save the changes.
The list of apps generating website pages is available in the "Structure" section of the Site app. You can also change the theme choice in that section for any app.