Drop the check for the theme name

This should not be necessary, given that we're validating that Furo
isn't added as an extension. Since that is enforced, it should not be
possible to misconfigure Furo and trigger a failure this way.

src/furo/__init__.py

@@ -168,10 +168,12 @@ def _html_page_context(
     context: Dict[str, Any],
     doctree: Any,
 ) -> None:
-    if app.config.html_theme != "furo":
-        return
-
-    assert isinstance(app.builder, StandaloneHTMLBuilder)
+    if not isinstance(app.builder, StandaloneHTMLBuilder):
+        raise Exception(
+            "Furo is being used with a non-HTML builder. "
+            "If you're seeing this error, it is a symptom of a mistake in your "
+            "configuration."
+        )
 
     if "css_files" in context:
         if "_static/styles/furo.css" not in context["css_files"]:
@@ -219,8 +221,6 @@ def _html_page_context(
 
 
 def _builder_inited(app: sphinx.application.Sphinx) -> None:
-    if app.config.html_theme != "furo":
-        return
     if "furo" in app.config.extensions:
         raise Exception(
             "Did you list 'furo' in the `extensions` in conf.py? "