Fix dark theme styling of roomheader cancel button

src/components/views/rooms/SimpleRoomHeader.js

@@ -26,7 +26,8 @@ export function CancelButton(props) {
 
     return (
         <AccessibleButton className='mx_RoomHeader_cancelButton' onClick={onClick}>
-            <img src="img/cancel.svg" width="18" height="18" alt="Cancel"/>
+            <img src="img/cancel.svg" className='mx_filterFlipColor'
+                width="18" height="18" alt="Cancel"/>
         </AccessibleButton>
     );
 }