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

Author: benjaoming

sphinx_rtd_theme/static/css/theme.css

@@ -431,7 +431,7 @@ footer
 +media($desktop-wider)
    .wy-nav-content-wrap
      background: rgba(0,0,0,.05)
-  .wy-nav-content
+   .wy-nav-content
      background: $section-background-color
 
 @media print