Hi, from what I understand, the following are the only properties you are able to control in pictureButton with themes: Border color, background color, font color.

From the Blue theme file:

"picturebutton":
{
"drawing":true,
"normal-state":
{
"border-color":"#999999",
"background-color":"#FFFFFF",
"text-font":{"color":"#000000"}
},
"hover-state":
{
"border-color":"#0078D7",
"background-color":"#E5F1FB",
"text-font":{"color":"#000000"}
},
"pressed-state":
{
"border-color":"#0060AC",
"background-color":"#CCE4F7",
"text-font":{"color":"#000000"}
},
"focused-state":
{
"border-color":"#0078D7",
"background-color":"#CCCCCC",
"text-font":{"color":"#000000"}
},
"disabled-state":
{
"border-color":"#CCCCCC",
"background-color":"#CCCCCC",
"text-font":{"color":"#999999"}
}
},