To set B4XFloatTextField's borders is quite simple.
B4XFloatTextField has TextField property, which retrieves a view (TextView in your case).
Any view has a field "layer", which has fields "borderWidth", "borderColor" etc.
But it seems this is a bad way, because B4XFloatTextField does not reserve a space for borders
Better to place B4XFloatTextField over panel (with borders), which is a little wider than B4XFloatTextField