Good afternoon, @imabusybee23.
At this time, there's no way to modify the background color to anything but the default white-and-green. However, dark-colored viewing modes (often referred to as Dark Mode) is becoming very popular among all web-based applications and is a commonly requested feature.
I'm preparing a feedback report to be sent to our Development Team on this matter. They're always looking to the opinions of users like yourself when determining how best to improve QuickBooks.
I can tell you from personal experience that popular browsers such as Google Chrome offer extensions which can reverse white backgrounds to darker colors for enhanced viewing pleasure. Head to your favorite search engine and search for a key phrase along the lines of Dark Mode Browser Extensions to check them out.
Should you have any additional questions or concerns, the Community will have you covered. Thanks for reaching out, best wishes.
