Add styling for cross-project-reference clipboard button

app/assets/stylesheets/pages/issuable.scss

@@ -88,3 +88,16 @@
     padding: 13px 0;
   }
 }
+
+.cross-project-reference {
+  text-align: center;
+  width: 100%;
+
+  .slead {
+    padding: 5px;
+  }
+
+  span, button {
+    background-color: $background-color;
+  }
+}