fix: highlighted lines were not highlighted (#37)

Author: Kocal

styles/code.styl

@@ -39,7 +39,7 @@ div[class*="language-"]
     width 100%
     line-height 1.4
     .highlighted
-      background-color rgba(0, 0, 0, 66%)
+      background-color rgba(255, 255, 255, 20%)
   pre, pre[class*="language-"]
     background transparent
     position relative