* changed html stylesheet to custom one

doc/doxygen/riot.doxyfile

@@ -125,7 +125,7 @@ HTML_OUTPUT            = html
 HTML_FILE_EXTENSION    = .html
 HTML_HEADER            = src/riot-header.html 
 HTML_FOOTER            = src/riot-footer.html
-HTML_STYLESHEET        = 
+HTML_STYLESHEET        = riot.css
 HTML_ALIGN_MEMBERS     = YES
 HTML_DYNAMIC_SECTIONS  = YES
 GENERATE_DOCSET        = NO