Dequeue theme stylesheets but keep widget styling on custom page template

You would have to enqueue them at the same time you dequeue all the other items, assuming your widgets have separate stylesheets.

You can’t selectively enqueue sub-portions of a stylesheet, and there is no method of identifying which stylesheets contain the widgets. Depending on the theme the widgets may not be in their own stylesheet, or even a single stylesheet

You’re almost certainly going to need to manually find the widget CSS rules and copy them into a separate stylesheet.

So can you do this by hand to make a specific theme work? Yes!

Can you do it in a generic way that works for every theme? No