Inconsistent serializer default parameters

[heuer]

Some serializers (i.e. SVG) use "transparent" for light modules, some serializers use "white" as default color for light modules and the border.

Maybe it would be nice if all serializers use "white" as default color for "light".
This would also mean somewhat larger files (e.g. SVG).

However, this would break backwards compatibility if users rely on the current behaviour.

Either keep the current behaviour or change all "light" defaults to "white" and release a new major version.