The check boxes employed in the Breeze plasma theme use a check mark and have much more shading than their counterparts in the widget theme. The widget theme also uses blue boxes over the black check marks. It would make greater visual sense for these two elements to match. Reproducible: Always
I agree so I'm confirming. However, the way checkboxes are constructed using the plasma theme svgs makes it quite difficult to achieve this. I think it uses a button background svg with the checkmark svg placed on top. In the main Breeze widget theme, the checkbox design does not use the same background as buttons. I've tried to think of ways to solve this in the theme but I haven't been able to.
Yes, the CheckBox is using the "widgets/button" element and draws a "widgets/checkmark" ontop of it. This could be changed to allow more freedom in the theme (with a fallback to the old style) but the fancy animation the Breeze theme has is probably not possible, at least not without making assumptions about the theme etc.
Perhaps new "checkbox_checked" and "checkbox_unchecked" elements in checkmarks.svg with the typical "normal", "pressed", "hover", and "focused" suffices? If those elements are present it renders them without the button background, if absent then it renders the "checkbox" element on the button background like it does today. Something similar would probably be needed for the radio buttons as well since the suffer from a similar problem. Of course, if we ever get to the point where we trust Qt Quick Controls Styles the way the Qt folks do for styling plasma widgets, then we already have a Breeze QtQuick Style waiting in the breeze repo that already closely matches main C++ widget theme, animations and all. :-)
checkmarks looks the same now