When installed, PMPro should already inherit your theme's font face/family. As you can see here, we are only adjusting the font-family on the textarea of PMPro: https://github.com/strangerstudios/paid-memberships-pro/blob/45b5849b13509575a5d9aff6d5a374c7848e46c4/css/frontend.css#L68.
We do adjust some of the font styling (bold, italic, size, etc...), as you stated above, in other places. You may need to use a more specific CSS selector to change the font styling on your theme. You could even copy the selectors we are using in the file above to your theme's stylesheet and make the necessary property adjustments. Hope this helps!