Final answer:
The term for a changing theme's color palettes and font families is a Theme Variant. It creates a new version of the theme with the same structure but different visual elements.
Step-by-step explanation:
The term for a variation of the theme using different color palettes and font families is d) Theme Variant. When you change the visual aspects of a theme, including the colors, font styles, and sometimes the layout, you are creating a new version of the theme or a theme variant. This approach allows for a consistent design structure while providing the flexibility to adapt to different contexts or preferences.