Remove unnecessary color in default stylesheet

resources/themes/default.qss

@@ -5,21 +5,12 @@
     using Qt::ArrowType do not change color when
     disabled, so we specify the color change ourselves.
 */
-QToolButton {
-    color: black;
-}
 QToolButton:disabled {
     color: gray;
 }
-QComboBox {
-    color: black;
-}
 QComboBox:disabled {
     color: gray;
 }
-QAbstractSpinBox {
-    color: black;
-}
 QAbstractSpinBox:disabled {
     color: gray;
 }