You can highlight weekends in the snippet with the week+day scales:
But it is not possible to do exactly the same thing as you show on the screenshot by using Gantt API.
But if you add the day scale, you will be able to highlight the weekends:
When the static background is enabled, Gantt generates the image depending on the
timeline_cell_class template. So, you see the same picture as if you don’t use the static background option.
If you want us to implement a feature to highlight the weekend days without using the day scale, please clarify why do you need that feature and why you don’t want to add the day scale.