How to change the font in posts from the Feeds feature?

On the pages of posts from the Feeds feature, the font is pulled from the Site Settings. The title font is used for the title, and the text font is used for the other text:

The font size is proportionally used from the Site Settings → Fonts → Size and Weight → Text Size (if you set a larger size, the text on the post page will also be larger, and vice versa):

Note that the ‘‘Text Size’’ parameter affects the whole website.