Update adding-custom-css.rst
Fixed typo
`where` -> `were`
@@ -42,7 +42,7 @@ A similar approach can be used to add JavaScript files::
.. note::
The Sphinx HTML options ``html_css_files`` and ``html_js_files``
- where added in Sphinx 1.8.
+ were added in Sphinx 1.8.
Unless you have a good reason to use an older version,
you are strongly encouraged to upgrade.
Sphinx is almost entirely backwards compatible.
