Event stroke color is theme's background color by default / provide option to override event stroke width and color

[olivewong]

Stroke color is set as the same as background, making thin lines difficult to see:

This gets assigned here: https://github.com/netzwerg/react-svg-timeline/blob/e7e9a7cfea432445710b5367e46bf05af43742f6/src/timeline/layers/Marks.tsx#L29

Feature request:

• allow passing in stroke color and stroke width in options for default events