latex rendering - transparent background instead of white background?

Dear PlantUML-Team

The one example on http://plantuml.com/ascii-math namely

@startuml
Bob -> Alice : Can you solve: $ax^2+bx+c=0$
Alice --> Bob: $x = (-b+-sqrt(b^2-4ac))/(2a)$
@enduml

shows a white background where I would expect transparency.

For all the other examples, the background color is carefully choosen to match the color of the node or note they are part of. That looks awesome.

Is it possible to make this one transparent instead of white? My intuition says yes, but can you implement it easily too?

thank you soo much for this awesome project!

best,

Stephan