Fix SASS syntax err, build theme.css for readthedocs#432

Author: benjaoming

sphinx_rtd_theme/static/css/theme.css

@@ -429,7 +429,7 @@ footer
     vertical-align: top
 
 +media($desktop-wider)
-   .wy-nav-content-wrap
+  .wy-nav-content-wrap
      background: rgba(0,0,0,.05)
   .wy-nav-content
      background: $section-background-color