Assistant in Qt 4.8 does not handle css import paths relative to
the includer.  This is fixed in Qt 4.8 commit b95750a275 (Assistant: Set
the url on created QNetworkReply objects., 2014-03-31).  It is unknown
whether there will be a further Qt 4.8 release containing that commit.

Use a CMake script to pre-replace the content prior to generating the
qch file.  An alternative workaround of moving the files or adding
"_static" to the import path did not seem to work for existing Qt 4.8
versions.

The bug was fixed in the Qt 5 branch before Qt 5.0. The Qt 5 assistant
renders this workaround'ed version correctly too.