Can I use gradients?

Yes, sparingly: petrol or orange, two steps at most, on backgrounds and visual elements, never in charts. Only the petrol gradient may fill text, in h1 and h2, and only the petrol gradient may colour an icon, on white, sand or light petrol.

A low-contrast gradient adds character without competing with the content, and in a chart flat colour reads more clearly. A gradient is only as strong as its lightest stop, and petrol's is the only one that holds contrast on text and icons.