OK. You are talking about font stroke and fill.
I don't think that's supported. See How To Add Text Outline With CSS - Kinsta®. for details on this but I Designer seems to reject the Webkit style instructions.
A simple solution is to use textShadow. Try the adding the following to the label's style properties:
textShadow : black 0px 0px 4px
https://docs.inductiveautomation.com/display/DOC81/Style+Reference#StyleReference-TextMenu