https://bugs.gentoo.org/543870#c11
--- a/doc/doxygen.cfg.in	2014-05-16 21:19:51.000000000 +0200
+++ b/doc/doxygen.cfg.in	2016-12-22 13:31:00.181040572 +0100
@@ -1069,7 +1069,7 @@
 # obsolete.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_STYLESHEET        = doxygen.css
+HTML_STYLESHEET        = @srcdir@/doxygen.css
 
 # The HTML_EXTRA_STYLESHEET tag can be used to specify an additional user-
 # defined cascading style sheet that is included after the standard style sheets