Theme
Change the application's colours from Menu ▸ Tools ▸ Theme.

You choose a primary colour and an accent colour. Suggested palettes are offered, but any combination is allowed. Click Save to apply.

The theme carries through to the web
Your chosen colours are also used by the web interface, so live timing screens match your club's look.
Light vs. dark is separate
This screen sets the accent colours. Whether a browser shows timing in light or dark is chosen per device on the web side — the app-hosted timing screens default to light, the online results/live site to dark. See Web Interface Settings.