You can use your web-browser's built-in Web Development Tools to Inspect the HTML element structure of the current page, as well as view the CSS that is effecting each element of that structure.
note: If you are using Windows then access that view is as simple as placing the mouse cursor over the main passage area of a running Twine story, clicking the Right mouse button to display a context menu and selecting the Inspect (or similarly named) menu item.
If you inspect the tw-story element and look at it's related CSS you will see that the default background-color and (foreground) color are set on this element, you would also notice that the default (passage) font is also set here.
You can change these defaults by placing CSS like the following within the Story Stylesheet area.
font-family: Georgia, serif;