fix($default-theme): indent-styled code is invisible (close: #609)

lib/default-theme/styles/code.styl

@@ -14,7 +14,8 @@
     line-height 1.4
     padding 1.25rem 1.5rem
     margin 0.85rem 0
-    background transparent
+    background-color $codeBgColor
+    border-radius 6px
     overflow auto
     code
       color #fff
@@ -36,7 +37,8 @@ div[class*="language-"]
     line-height 1.4
     .highlighted
       background-color rgba(0, 0, 0, 66%)
-  pre
+  pre, pre[class*="language-"]
+    background transparent
     position relative
     z-index 1
   &::before