...
This main site theme applies to your entire site, except for the Refined Pages, spaces and projects that have their own specific theme set (see the paragraph below).
Go to the site Site structure.
Click on the name of your current theme. located in the In the home page item which has your site’s name in it(located at the top of the structure, and has the same name as your site), click “Site theme: (current theme name)”.
Select a theme.
...
Apply a theme to a specific Refined Page, space, or project
...
In the site structure, click on the item's
three dots > Settings.Click on the theme name.
Click the Change theme button.
Select a theme.
...