Is it OK to mix dark and light backgrounds in an app?

Yes of course. Backgrounds (and colors in general) are used for different purposes, including highlighting elements and whole sections. A completey different background will create a sense of alert and users will pay special attention to that.

1. Should backgrounds be lighter or darker?
2. Should coding be white or black background?
3. What are the rules for Dark theme?
4. Is it OK to use dark mode all the time?

Should backgrounds be lighter or darker?

Light backgrounds will be more appropriate for the elderly and visually impaired, dark backgrounds may be more attractive for a younger audience or a target group that particularly appreciates the professionalism and elegance of the websites they browse.

Should coding be white or black background?

Most developers use a black screen with color-coded syntax to reduce eye strain. As Toptal developer Kevin Bloch puts it: “A black background reduces eye strain and makes automatic color coding easier to read, which makes code much faster to understand at a glance.”

What are the rules for Dark theme?

Dark theme surfaces must be dark enough to display white text. They should use a contrast level of at least 15.8:1 between text and the background. This ensures that body text passes WCAG's AA standard of at least 4.5:1 when applied to surfaces at the highest (and lightest) elevation.

Is it OK to use dark mode all the time?

Dark mode may increase eye strain in brightly-lit conditions. Dark mode can cause halation for individuals with myopia or astigmatism, making text less readable for them. Dark mode may lower reading comprehension and focus. Dark mode does not improve battery life on older devices without OLED screens.