Style Notes are a short description of the look and feel you want. We apply them every time an app is built or updated, so you don't have to re-type "dark, rounded corners, playful" into every prompt.
There are two sets. Account style notes under Account → Style notes apply to everything you build. App style notes in an app's own settings apply to just that app, and they take priority over your account ones. Leave an app's notes empty and it follows your account.
Either way, you can pick a ready-made style, answer five quick questions and have the notes written for you, or just describe the look yourself in up to 1,000 characters. However you fill it in, you can edit the text afterwards. Press Save when you're done.
Be specific about things you'd notice if they were wrong: colors, mood, typography, spacing, corners, how much personality it should have. "Make it look modern" doesn't steer anything. Keep it to how apps look, not what they do.
New notes apply going forward. They won't restyle an app you've already built, but the next change you ask for will bring the new style with it.