Don't make nav links white to prevent then from not showing up when the images are not present.

[SVN r39384]

doc/html/boostbook.css

@@ -176,7 +176,6 @@
     
     .spirit-nav a
     {
-        color: white;
         padding-left: 0.5em;
     }
     

doc/src/boostbook.css

@@ -176,7 +176,6 @@
     
     .spirit-nav a
     {
-        color: white;
         padding-left: 0.5em;
     }