Expose fit function style

[markov00]

Describe the solution you'd like
The user may desire to style differently the fitted values depending on their use cases.
We currently hardcoded some values, like the dashed style and the opacity value of the area.
We should expose, through the theme, a way to add a custom style for those rendered lines/areas/points.

Describe alternatives you've considered
We probably don't want to include a new fit function to linearly connect all points with a straight line and no area opacity.
Exposing the style is way come useful and the user can also reuse that style in different fitting functions.