Include custom CSS separately instead of overriding theme CSS

May help with https://github.com/DFHack/dfhack/issues/1578#issuecomment-635512324

conf.py

@@ -187,7 +187,7 @@ all_keybinds_documented()
 # -- General configuration ------------------------------------------------
 
 # If your documentation needs a minimal Sphinx version, state it here.
-needs_sphinx = '1.3'
+needs_sphinx = '1.8'
 
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
@@ -286,7 +286,6 @@ todo_include_todos = False
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
 html_theme = 'alabaster'
-html_style = 'dfhack.css'
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the
@@ -337,6 +336,10 @@ html_domain_indices = False
 # If false, no index is generated.
 html_use_index = False
 
+html_css_files = [
+    'dfhack.css',
+]
+
 # -- Options for LaTeX output ---------------------------------------------
 
 # Grouping the document tree into LaTeX files. List of tuples

docs/styles/dfhack.css

@@ -1,6 +1,3 @@
-/* make sure to sync this with the base theme's css filename */
-@import url("alabaster.css");
-
 /* Keybinding CSS from the DF wiki; applies to :kbd:`` directives.
  * Use this directive for all keypresses, to make them look like keys.
  */