You can modify the background and text colors of the highlighted region to your liking. The default colors were selected to fit with Ao3's usual colorscheme.

If you're using the default Archive theme, you don't need to do anything to the "Page body BG color" and "Page body text color" settings. If you're using a theme that changes the background color and/or text color of the site, input whatever values that theme provides.

Thanks to pedanther for CSS help!