Update ui.css.scss

The ui-dialog is not well rendered in version 2.1 compared to the same ui-dialog in version 2.0 : the cross on the top right side is not well positioned and the rounded corners are not rounded.

The problem is that the css didn't change between v2.0 and v2.1.

I modified the ui.css.scss to get the cross back in the good place but I didn't do anything for the rounded corners.