Everything about how the app looks sits together in Settings under Appearance, on both platforms: appearance, themes, accent color and text size. Appearance, the accent and the text size are there on every plan; the themes are part of the Pro and Lifetime plans.
Appearance
Light, Dark, or System, which follows your device and flips automatically when it does. Notes, dialogs and the drawing canvas all follow along, so a note written at night reads fine in the morning. The theme picks next to it choose which theme paints the app in each mode, on Pro and Lifetime: the Light theme and Dark theme grids on desktop, the Themes row on mobile.
Accent color
Seventeen options: Amber, Red, Orange, Yellow, Lime, Green, Emerald, Teal, Cyan, Sky, Blue, Indigo, Violet, Purple, Fuchsia, Pink and Rose. The accent shows up in buttons, selections and highlights across the app, mini apps included, on top of whichever theme you use.
What follows you
Appearance, the accent and your theme picks all sync with the rest of your settings, so switching your phone to dark switches your desktop too, and the two agree on how Grape looks without setting it twice. Text size is the one that stays per device, since a phone and a desk monitor want different sizes.