Add possibility to change font sizes and colors

[andy-arbeit]

Is your feature request related to a problem? Please describe.
I would like to change some font sizes and colors, especially in the chart-container (size of shown day). I cannot solve this with css.

Describe the solution you'd like
Can you add the possibility to change font sizes of the main and the chart-container? Alternatively I would like to get the css-object to change it.

Additional context
Add any other context or screenshots about the feature request here.