There is no layout tool to make a boarder around controls this with the native Ai2 controls. There are ways to 'simulate' a border by using a Canvas with a background image and using sprites as 'buttons' put inside borders drawn on the background image.. AI2's layout controls are not very sophisticated, as a result, users seem to find all kinds of ways of designing appropriate screens using a combinations of labels, buttons, images in buttons and using labels and buttons (without text) as 'spacers.'
A simple way to simulate a border around a button is to place an image in the button that has a small 'border' on the image. Actually, that is a border inside the label. Try some stuff. To do things like this you need a compiler with fancier layout controls like Eclipse or Android Studio or to design the feature yourself in your 'own' version of AI2 .... the code is opensource.
http://appinventor.mit.edu/appinventor-sources/
Regards,