## conf.py

# These folders are copied to the documentation's HTML output
html_static_path = ['_static']

# These paths are either relative to html_static_path
# or fully qualified paths (eg. https://...)
html_css_files = [
    'css/custom.css',
]

/** css/yourtheme.css **/

/* This line is theme specific - it includes the base theme CSS */
@import '../alabaster.css';  /* for Alabaster */
/*@import 'theme.css';       /* for the Read the Docs theme */

body {
    /* ... */
}

## conf.py

html_style = 'css/yourtheme.css'