Open Theme > General and edit the colour rows:

• Primary Color and Primary Color - Contrast Color: main brand colour and the text colour used on top of it (buttons, active dates, highlights, cart overlays). Default green #00d95b with white text if empty.
• Secondary Color and Secondary Color - Contrast Color: secondary accent; falls back to the primary colour if empty.
• Footer Color and Footer Color - Contrast Color: footer background and footer text/link colour.
• Menu Color, Menu Color - Contrast Color, Menu Color - Focus Color: header navigation background, text, and hover/active colour (focus defaults to the primary colour).

Enter hex codes and save each row; the site picks up the new colours on the next page load. The primary colour is also used as the browser theme colour on mobile.