oh man it's been a while,
1) there is a flag you can include to make the window transparent, and another to set the value of transparency.
2) there's some kind of statement structure similar to if/else where you can see if the value is less than 10, less than 30, else (i forget what it is... sorry im at work and haven't looked at this in some time)
3) nope, those come straight from the server side... though, you can implement something i had been planning and never got around to! You can make a hotbar mask that is square inside and out, and you will have to layer this ontop of the real hotbar that contains your rounded buttons