Open the menu (≡) at the top left of the Notes screen → "Theme Color", choose the theme you like and tap "Apply". There are 23 colors in total. "Good Night Blue", "Champagne Navy" and "Monotone Black" are dark themes; choosing one of them makes the whole screen dark.
* The app does not follow your device's dark mode setting. To get a dark screen, choose a dark theme.
* 10 of the 23 colors require the Premium Service. The 13 free colors are "Default", "Simple Blue", "Simple Gray", "Monotone Black", "Natural Pink", "Softly Pink", "Fresh Orange", "Cream Yellow", "Aqua Yellow", "Matcha Green", "Good Night Blue", "Sleepy Purple" and "Champagne Navy".