Could you please state, how you have tried to achieve your goal? I have just been testing and there seems to be no difficulty at all to paint the legend transparent:
I played around a bit and came to the conclusion that this behaviour is down to the silver type of front panel elements. I could not find a way to circumvent this lack of transparency setting of the silver controls. Is there a chance for you to switch to modern or classic elements? The modern graph still shows a border around the legend that cannot be made transparent. But with the classic graph that border will also disappear. I am not sure about the cursor table border lines in black - I can't find a menu-based option to make those disappear as well. Also, the background of the cursor movement buttons cannot be painted transparent in the classic graph, although the buttons themselves can be. Try to dig through the properties of the graph in VI server, maybe there is something you cannot touch via menus.
You're right about the "Silver" type.
About the black border in "Classic" - you should press "tab" while coloring and click "T" again so that both colors would be Transparent. Then click on the line, and it will become transparent.
Actually you CAN change the color for the Silver style. You have to customize it first.
In the Customize Mode, select the Cursor Legend, from menu bar => Edit => Customize Control. Now you get the 2nd customize dialog window. There are 7 layers, you can resize the 1st layer and then move, change color to other layers.
Thank you for the tip! I was trying to customize it, but the right click wasn't giving the opportunity. When I tried to customize it from "Edit", it worked.