doc/style: use default font color specified by user

* it's bad to specify only foreground color:
  http://www.w3.org/QA/Tips/color
* some people prefer dark background
* `color:  #111' is mostly the same as `color: black',
  which is the default of almost all popular browsers.

so it's preferable to delete `color:  #111', rather than adding
`background-color: white'.

doc/style.css

@@ -12,7 +12,6 b' body {'
     margin: 0;
     padding: 0;
     font-family: sans-serif;
-    color: #111;
 }
 
 .document {
@@ -72,7 +71,7 b' a:link { color: #00b5f1; }'
 a:visited { color: #5c9caf; }
 a:link.toc-backref, a:visited.toc-backref {
     text-decoration: none;
-    color: #111;  /* `inherit' is not supported by IE6 */
+    color: inherit;  /* NOTE: `inherit' is not supported by IE6 */
 }
 
 div.admonition, div.attention, div.caution,