It's not clear to me whether a designer would adhere to Polaris specs if they were developing a custom theme for the Shopify store. Would they base it off of the file structure in Shopify sections, snippets, etc. or does Polaris provide something better?
Polaris is used only for Apps since it provides a similar style to the Shopify admin.
That said you can use it in a theme if you want to use similar styled elements as the admin, but coding a whole theme using Polaris will look a little back-end if you understand what I mean.
The idea of Polaris is to make Apps look similar to the Shopify admin design so that users are not faced with the color of the rainbow when they open a random app in their shopify panel.
All Polaris design guidelines and components are written for Apps and not themes.
So yes, you can use it on a theme, but No I don't recommend it.