CheckBox: support styling indeterminate state of tri-state check boxes (issue #919)

flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemePreviewSwitches.java

@@ -672,7 +672,7 @@ class FlatThemePreviewSwitches
 				}
 				@Override
 				public boolean isSelected() {
-					return isStateSelected();
+					return isStateSelected() || isStateIndeterminate();
 				}
 			} );