Add pressed appearance for checked checkboxes and radio buttons

There was a pressed appearance for unchecked checkboxes and radio
buttons, but we were missing one for their checked states. This
commit simply adds the same effect: a slight and subtle darkening
while pressed.

BUG: 451428
FIXED-IN: 5.26

kstyle/breezehelper.cpp

@@ -800,7 +800,7 @@ void Helper::renderCheckBoxBackground(QPainter *painter,
 
     case CheckPartial:
     case CheckOn:
-        painter->setBrush(transparent);
+        painter->setBrush(transparent.darker(sunken ? radioCheckSunkenDarkeningFactor : 100));
         painter->drawRoundedRect(frameRect, radius, radius);
         break;
 
@@ -953,7 +953,7 @@ void Helper::renderRadioButtonBackground(QPainter *painter,
         painter->drawEllipse(frameRect);
         break;
     case RadioOn:
-        painter->setBrush(transparent);
+        painter->setBrush(transparent.darker(sunken ? radioCheckSunkenDarkeningFactor : 100));
         painter->drawEllipse(frameRect);
         break;
     case RadioAnimated: