add chessground workaround in ui/learn css - closes #8286

moved from
hi-ogawa@832086f

Author: ornicar

ui/learn/css/_board.scss

@@ -151,3 +151,20 @@
     content: '1';
   }
 }
+// Workaround for chessground 4.4 used in `ui/learn` page.
+// This selector has no effect for chessground 7.11.0 used in other pages.
+cg-container > cg-board > svg {
+  overflow: hidden;
+  position: absolute;
+  top: 0px;
+  left: 0px;
+  width: 100%;
+  height: 100%;
+  pointer-events: none;
+  z-index: z('cg__svg.cg-shapes');
+  opacity: 0.6;
+
+  image {
+    opacity: 0.5;
+  }
+}