Indoors, the room decides: the same baby wears a short-sleeve bodysuit in a warm flat and a sweater and warm socks in a cold one. The app's own answers, room by room.
24°C and up: short-sleeve bodysuit. Warm at home — one light layer is likely plenty. Bare legs are fine. 21–23°C: short-sleeve bodysuit and socks. Comfortable room — a light top and socks. 18–20°C: long-sleeve bodysuit, pants and socks. A normal room — one light layer head to toe. 15–17°C: long-sleeve bodysuit, pants, socks and light sweater. Cool at home — add a light sweater. Under 15°C: long-sleeve bodysuit, warm pants, warm socks and warm sweater. Cold room — warm layers all round. Warming the room a touch helps most.
Indoors, a baby wears one layer less than outside at the same temperature: no wind, no hat, and a parent nearby to add or take off a layer.
For sleep, the sack takes over: see the TOG chart for what to wear to bed in each room.
Check the neck — warm but not sweaty means just right.