All Nimbus components that paint focus or have rounded corners should be non-opaque by default. This is because when you set the components backgound color on text field for example the backgound color should be the color of the area behind the text, not the color of the area around the outside of the border. The area in the corners and around the outside of the border should just be showing though to the parent component, ie transparent = non-opaque.
This means that it is down to the components to decide what its backgound color should map to, this is undefined for components like Button and Combo whos backgound is a complex shaded fill and not a single color. Its also undefined for components like checkbox and radiobutton that have no backgound as they are only a icon and text. For textfields they should map there backgound color to the white area behind the text.