Store stylesheets as an instance variable of HTML builder

So far, CSS files are stored as a class variable of HTML builder.
Not to have status globally, this changes it to an instance varable.

sphinx/application.py

@@ -1036,16 +1036,14 @@ def add_stylesheet(self, filename, **kwargs):
            Allows keyword arguments as attributes of link tag.
         """
         logger.debug('[app] adding stylesheet: %r', filename)
-        from sphinx.builders.html import StandaloneHTMLBuilder, Stylesheet
         if '://' not in filename:
             filename = posixpath.join('_static', filename)
         if kwargs.pop('alternate', None):
             warnings.warn('The alternate option for app.add_stylesheet() is deprecated. '
                           'Please use rel="alternate stylesheet" option instead.',
                           RemovedInSphinx30Warning)
             kwargs['rel'] = 'alternate stylesheet'
-        css = Stylesheet(filename, **kwargs)
-        StandaloneHTMLBuilder.css_files.append(css)
+        self.registry.add_css_files(filename, **kwargs)
 
     def add_latex_package(self, packagename, options=None):
         # type: (unicode, unicode) -> None

sphinx/builders/html.py

@@ -248,15 +248,20 @@ class StandaloneHTMLBuilder(Builder):
     # This is a class attribute because it is mutated by Sphinx.add_javascript.
     script_files = ['_static/jquery.js', '_static/underscore.js',
                     '_static/doctools.js']  # type: List[unicode]
-    # Ditto for this one (Sphinx.add_stylesheet).
-    css_files = CSSContainer()  # type: List[Dict[unicode, unicode]]
 
     imgpath = None          # type: unicode
     domain_indices = []     # type: List[Tuple[unicode, Type[Index], List[Tuple[unicode, List[List[Union[unicode, int]]]]], bool]]  # NOQA
 
     # cached publisher object for snippets
     _publisher = None
 
+    def __init__(self, app):
+        # type: (Sphinx) -> None
+        super(StandaloneHTMLBuilder, self).__init__(app)
+
+        # CSS files
+        self.css_files = CSSContainer()  # type: List[Dict[unicode, unicode]]
+
     def init(self):
         # type: () -> None
         self.build_info = self.create_build_info()
@@ -269,6 +274,7 @@ def init(self):
 
         self.init_templates()
         self.init_highlighter()
+        self.init_css_files()
         if self.config.html_file_suffix is not None:
             self.out_suffix = self.config.html_file_suffix
 
@@ -331,6 +337,11 @@ def init_highlighter(self):
         self.highlighter = PygmentsBridge('html', style,
                                           self.config.trim_doctest_flags)
 
+    def init_css_files(self):
+        # type: () -> None
+        for filename, attrs in self.app.registry.css_files:
+            self.css_files.append(Stylesheet(filename, **attrs))  # type: ignore
+
     @property
     def default_translator_class(self):
         # type: () -> nodes.NodeVisitor
@@ -1332,6 +1343,7 @@ def init(self):
         self.templates = None   # no template bridge necessary
         self.init_templates()
         self.init_highlighter()
+        self.init_css_files()
         self.use_index = self.get_builder_config('use_index', 'html')
 
     def get_target_uri(self, docname, typ=None):

sphinx/registry.py

@@ -65,6 +65,9 @@ def __init__(self):
         #: autodoc documenters; a dict of documenter name -> documenter class
         self.documenters = {}           # type: Dict[unicode, Type[Documenter]]
 
+        #: css_files; a list of tuple of filename and attributes
+        self.css_files = []             # type: List[Tuple[unicode, Dict[unicode, unicode]]]
+
         #: domains; a dict of domain name -> domain class
         self.domains = {}               # type: Dict[unicode, Type[Domain]]
 
@@ -412,6 +415,9 @@ def add_autodoc_attrgetter(self, typ, attrgetter):
         # type: (Type, Callable[[Any, unicode, Any], Any]) -> None
         self.autodoc_attrgettrs[typ] = attrgetter
 
+    def add_css_files(self, filename, **attributes):
+        self.css_files.append((filename, attributes))
+
     def add_latex_package(self, name, options):
         # type: (unicode, unicode) -> None
         logger.debug('[app] adding latex package: %r', name)