Several default themes are using system fonts. This is a great practice since they have great language support and it avoids loading several font files. A disadvantage is that they are often not very similar on different operating systems (OSs).

An interesting alternative would be to use a modern font stack. It has the same advantages as a system font stack, but in addition they are chosen to match better between different OSs.

The modern font stacks website showcases several ideas. For example, the Neo-Grotesque stack could be a great alternative to where the theme now uses system fonts. The GitHub repo of the project demonstrates how this stack looks in different OSs.