Revert [r9410] "Prevent the 'theme' default overriding 'theme_url'."
As documented in docutils/docs/user/config.html#override,
"Client programs which specify defaults that override other settings must
do the overriding explicitly". This affects not only "theme"/"theme_url",
but also "stlesheet"/"stylesheet_path" (HTML and LaTeX), and
"footnote_references"/"trim_footnote_reference_space".
Exlicitly override "theme" in the S5 functional test setup instead.
git-svn-id: http://svn.code.sf.net/p/docutils/code/trunk@9542 929543f6-e4f2-0310-98a6-ba3bd3dd1d04