[libxml++] libxml++ reference documentation inheritance diagram images have black background

[Jim Hodapp <james hodapp gmail com>]

Both on the website, and the locally run (via the sourcecode) versions 
of the doxygen libxml++ reference documentation have black backgrounds 
for the class inheritance diagrams when viewed with [at least] Firefox 
v3.5. This makes these inheritance diagrams pretty worthless. I imagine 
it has to do with the images being generated with an transparent 
background and Firefox setting it to be black for some reason.

Thanks,

Jim Hodapp