Go to “Sites” and then select your website where you want to change the theme. Click on the “Edit” button on the website!
Click on the tab “Template” in the website details.
Click on the “Template” drop down menu and select a theme for your website.
If you have selected a theme, you can click “SAVE”. Immediately the new webpages theme is applied on your site. To test you can now look at your website!