html_static_path doesn't look for css files relative to it

[rtedwards]

Expected Behavior

The instructions from Custom CSS Stylesheets indicates that custom.css will be activated with the following setup

1. Create a CSS stylesheet in _static/css/custom.css, and update the CSS rules as desired.
2. Attach the stylesheet to your Sphinx build. Add the following to conf.py:

html_static_path = ['_static']

html_css_files = [
    'css/custom.css',
]

Actual Behavior

Custom style sheets are only activated when using the full path

html_static_path = ['_static']

html_css_files = [
    '_static/css/custom.css',
]