Use <a> color for blockquote.notice border (mmistakes#3140)

Close mmistakes#3068

_sass/minimal-mistakes/_notices.scss

@@ -53,6 +53,10 @@
     }
   }
 
+  @at-root #{selector-unify(&, "blockquote")} {
+    border-left-color: mix(#000, $notice-color, 10%);
+  }
+
   code {
     background-color: mix($background-color, $notice-color, $code-notice-background-mix)
   }