Twine's default styling using the Harlowe 3.1.0 Style Format (a set of styling rules and options), with a black background and white text.
You can customize how your story looks if you know a little CSS. While Style Formats are beyond the scope of this tutorial, you can access the style sheet for your story in the Passages View title menu. Select "Edit Story Stylesheet" and enter the changes you would like. Any easy change, for example, is is background color. For example, if you'd like your background to be blue, you can you the term blue or enter a more specific hex code to get the look you'd like.
Not sure how to use CSS? Don't spin your wheels! There are multiple tutorials on the internet.