The Starter theme actually uses a more modern stack than the other themes, opting for Rollup instead of Gulp. Since it’s expected that the styling will be modified by the developer making the custom theme, the styling hasn’t been drastically updated.
The Starter theme is a better candidate for creating a custom theme. It includes more conveniences out of the box. We have a tutorial and video on how to use it to get set up for custom theme dev here: