refactor($theme-default): clean all `.content` class names

packages/@vuepress/theme-default/styles/code.styl

-.content
+{$contentClass}
   code
     color lighten($textColor, 20%)
     padding 0.25rem 0.5rem
@@ -12,7 +12,7 @@
       &.inserted
         color $accentColor
 
-.content
+{$contentClass}
   pre, pre[class*="language-"]
     line-height 1.4
     padding 1.25rem 1.5rem

packages/@vuepress/theme-default/styles/config.styl

+$contentClass = '.theme-default-content'

packages/@vuepress/theme-default/styles/index.styl

+@require './config'
 @require './code'
 @require './custom-blocks'
 @require './arrow'
 @require './wrapper'
 @require './toc'
 
-$contentClass = '.theme-default-content'
-
 html, body
   padding 0
   margin 0

packages/@vuepress/theme-default/styles/mobile.styl

@@ -31,7 +31,7 @@ $mobileSidebarWidth = $sidebarWidth * 0.82
 @media (max-width: $MQMobileNarrow)
   h1
     font-size 1.9rem
-  .content
+  {$contentClass}
     div[class*="language-"]
       margin 0.85rem -1.5rem
       border-radius 0