Better documentation for changing styles

I am unsure on how to change styles of my charts as described in the README. Does the style key need to be in the YAML file as well? I tried changing the stroke color of a uncompleted task by adding the following code in my chart YAML file:

style:
  tasks:
    uncompleted:
      stroke:
        paint: red

But the compiler throws an error: dictionary does not contain key "gridlines". Would I need to map out all the keys in the default_style variable just to change a single value?

It would be very helpful if an example on how to change styles correctly could be added 😄