LibWeb: Let about:blank use dark color schemes

Opt empty about:blank documents into both light and dark supported
color schemes. This covers both navigation-created about:blank documents
and the initial about:blank document created with a browsing context, so
dark mode can choose the dark Canvas color before page content arrives.

Libraries/LibWeb/DOM/Document.cpp

@@ -2492,6 +2492,11 @@ Optional<Vector<String> const&> Document::supported_color_schemes() const
     return m_supported_color_schemes;
 }
 
+void Document::set_supported_color_schemes(Vector<String> supported_color_schemes)
+{
+    m_supported_color_schemes = move(supported_color_schemes);
+}
+
 // https://html.spec.whatwg.org/multipage/semantics.html#meta-color-scheme
 void Document::obtain_supported_color_schemes()
 {

Libraries/LibWeb/DOM/Document.h

@@ -372,6 +372,7 @@ public:
     void set_visited_link_color(Color);
 
     Optional<Vector<String> const&> supported_color_schemes() const;
+    void set_supported_color_schemes(Vector<String>);
     void obtain_supported_color_schemes();
 
     void obtain_theme_color();

Libraries/LibWeb/DOM/DocumentLoading.cpp

@@ -97,6 +97,7 @@ static WebIDL::ExceptionOr<GC::Ref<DOM::Document>> load_html_document(HTML::Navi
     if (document->url_string() == "about:blank"_string
         && navigation_params.response->body()->length().value_or(0) == 0) {
         TRY(document->populate_with_html_head_and_body());
+        document->set_supported_color_schemes({ "light"_string, "dark"_string });
         HTML::HTMLParser::the_end(document);
     }
 

Libraries/LibWeb/HTML/BrowsingContext.cpp

@@ -278,6 +278,7 @@ BrowsingContext::BrowsingContextAndDocument BrowsingContext::create_a_new_browsi
 
     // 19. Populate with html/head/body given document.
     populate_with_html_head_body(*document);
+    document->set_supported_color_schemes({ "light"_string, "dark"_string });
 
     // 20. Make active document.
     document->make_active();