Twine's default styling uses 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 (cascading style sheets). 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 in CSS. Note that that the formatting used below is for the default Harlowe 3.1.0 Story Format (see Overview). If you change your Story Format, the markup and HTML might be different.
The image above shows the default styling for your story: black background and white text.
Any easy change is background color. For example, if you'd like your story passage background to be blue and your text to be black, you can specify through CSS both light blue or black options. In the style sheet you specify that you'd like these changes to affect your whole story by calling "tw-story" and using curly brackets to specify those changes.
In the style sheet above, we are saying that the whole story (tw-story) should have a background of light blue and a text color of black.
When you select Play, you'll see the style changes appear immediately.