Once a tenant and site are created and/or a theme is manually created you may want to assign a theme to a site. A theme is automatically assigned when during a site creation but what if you want to assign a theme to already created site.
See this blog to create a tenant and site and different approaches of creating a theme.
Select any Partial design and open experience editor select Experience Accelerator option.
Select Theme option. Change the theme.
This will also the change the theme in the selected site – /sitecore/content/<<tenant folder>>/<<tenant>>/<<site folder>>/<<site>>/Presentation/Page Designs
To change the theme for a specific site. Navigate to the site/Presentation/Page Designs
for e.g.:- /sitecore/content/<<tenant folder>>/<<tenant>>/<<site folder>>/<<site>>/Presentation/Page Designs
In the Styling section change the Theme field
Hope this helps.