If you installed it. Goto admin in side bar. Then click on Theme in admin side bar.
Choose theme in your last st and ensure it is enabled Install Instructions Here
You can choose it to be selectable and or default.
To switch themes click on your avatar in header choose person icon and preferences. Then scroll right if on mobile and choose interface.
Optional there is a theme-component that adds a theme switcher in the sidebar like here on Meta
There is an install button just enter your site’s url