Students often request a night mode or dark mode.
A common approach for web applications like ours (vs. an iPhone/Android app) is to use a browser extension / add-on that provides night mode functionality for any web page.
If you find an extension that works well with zyBooks, email email@example.com to let us know. We might provide recommendations here in the future based on such feedback.
We'll also keep the request in mind for a built-in night mode.