Draft: fix tray construction mode button for Light and Dark themes (#17886)

Fixes #17869.

src/Mod/Draft/DraftGui.py

@@ -228,11 +228,14 @@ class DraftToolBar:
 
     def _pushbutton(self,name, layout, hide=True, icon=None,
                     width=None, checkable=False, square=False):
-        button = QtWidgets.QPushButton(self.baseWidget)
-        button.setObjectName(name)
         if square:
-            button.setMaximumSize(QtCore.QSize(button.height(), button.height()))
-            button.setFlat(True)
+            button = QtWidgets.QToolButton(self.baseWidget)
+            if width is not None:
+                button.setFixedHeight(width)
+                button.setFixedWidth(width)
+        else:
+            button = QtWidgets.QPushButton(self.baseWidget)
+        button.setObjectName(name)
         if hide:
             button.hide()
         if icon:
@@ -488,7 +491,7 @@ class DraftToolBar:
         self.setStyleButton()
         self.constrButton = self._pushbutton(
             "constrButton", self.toptray, hide=False, icon='Draft_Construction',
-             checkable=True, square=True)
+            width=self.styleButton.sizeHint().height(), checkable=True, square=True)
         self.constrColor = QtGui.QColor(self.paramconstr)
         self.autoGroupButton = self._pushbutton(
             "autoGroup", self.bottomtray,icon=":/icons/button_invalid.svg",