Its already there: dashed or dotted.
Go to menu: Tools > Option
Then: Editor > Display > Colors
There is a list of colors (text, comment, keyword, number....)
Scroll this all the way down.
You find "Outline Colors"
Select each level, and set color and style as you like it.
You want to change the setting for the "vertical mark", which is the vertical line.
The other colors affect the keyword.
You can increase/decrease the amount of colors used. Once the highest configured color is reached, it restarts at level 1.