In both Light and Dark modes, the contrast between the text and background needs to be clear enough to offer good readability. Similarly, the contrast between backgrounds and focus blocks needs to be small enough to keep the experience visually comfortable.
Dark mode offers particular advantages, especially at night, so it's important that apps are optimized for it. When on, backgrounds, menus, and other UI elements turn black or dark gray to reduce glare for users, but they can also make images and other content visually jarring. Be sure to test apps and features in both Light and Dark modes before release.