Go back to single colon pseudo elements.

This is for backwards compatibility with the browsers we support in v3.

less/carousel.less

@@ -156,12 +156,12 @@
   }
 
   .icon-prev {
-    &::before {
+    &:before {
       content: "\2039";// SINGLE LEFT-POINTING ANGLE QUOTATION MARK (U+2039)
     }
   }
   .icon-next {
-    &::before {
+    &:before {
       content: "\203a";// SINGLE RIGHT-POINTING ANGLE QUOTATION MARK (U+203A)
     }
   }