Here topic line, means topic border line and link line which connects to it's parent topic.
A. Topic link line style
There are six styles of topic link line: no line,straight line,arc,curve,elbow,angle.
B. Topic link line thickness
You can set link line thickness between central topic and main topic. Subtopic link line thinkness is alway1 .
C. Topic border line and link line color
Select topic, then click menu item "Formation->Line->Line color" to set line color.