Changing backgroundColor of a button changes its style ?

[Alex Tweedly]

1. create a new stack

2. drag a 'standard button' from the toolbar onto it

(you now have a nicely rounded-corner button)

3. set its script to

on mouseup
    set the backgroundcolor of me to "200,50,50"
end mouseup

4, click on 'browse mode' so the stack is "live"

4a. observe the button still has nicely rounded corners :-)

5. click on it

  the backgroundcolor changes to red

AND the rounded corners disappear - it's now a rectangular button.

What am I doing wrong or missing ?

(LC 9.0.0, MacOS 10.13.2)

Thanks

Alex.