Setting your device to dark mode means that it will display white text on a dark background. Dark mode is intended to reduce blue light exposure and help with eye strain that comes with prolonged screen time.
To enable dark-mode, click the "gear icon" in the top right corner and find "Themes" in the dropdown menu. Click "View more" and tick the box next to "Auto dark-mode" at the bottom of the "Choose a theme" window.
Tip: You can also customize your email with accent colors, backgrounds, images and more from the "Choose a them" window. Make sure to click "Apply" when you are happy with your selections.