MsgBoxAsync uses the alert class of JavaFx. This class allows to define a style (.css) : size color, background color ...
However to my knowledge it is not possible to access this possibility from B4X.
A solution would be to modify the java source code of the MsgBoxAsync function.
But the easiest way is probably to use from B4x "B4XDialog - Custom dialogs" to design a box to your needs.