LibWebView: Add a standard "look like the OS" style sheet

It's quite common to want the web-view to match the native OS widgets,
so let's make that easy to do.

Userland/Libraries/LibWebView/CMakeLists.txt

@@ -10,11 +10,20 @@ set(SOURCES
     WebSocketClientAdapter.cpp
 )
 
+embed_as_string_view(
+    "NativeStyleSheetSource.cpp"
+    "${CMAKE_CURRENT_SOURCE_DIR}/Native.css"
+    "NativeStyleSheetSource.cpp"
+    "native_stylesheet_source"
+    NAMESPACE "WebView"
+)
+
 set(GENERATED_SOURCES
     ../../Services/RequestServer/RequestClientEndpoint.h
     ../../Services/RequestServer/RequestServerEndpoint.h
     ../../Services/WebContent/WebContentClientEndpoint.h
     ../../Services/WebContent/WebContentServerEndpoint.h
+    NativeStyleSheetSource.cpp
 )
 
 serenity_lib(LibWebView webview)

Userland/Libraries/LibWebView/Native.css

@@ -0,0 +1,35 @@
+/*
+ * Stylesheet designed to be used as the user stylesheet by apps that want their
+ * web content to look like the native GUI.
+ */
+
+html {
+    background-color: -libweb-palette-base;
+    color: -libweb-palette-base-text;
+}
+
+input, textarea {
+    border-color: -libweb-palette-threed-shadow1;
+}
+
+button, input[type=submit], input[type=button], input[type=reset] {
+    background-color: -libweb-palette-button;
+    border-color: -libweb-palette-threed-shadow1;
+    color: -libweb-palette-button-text;
+}
+
+button:hover, input[type=submit]:hover, input[type=button]:hover, input[type=reset]:hover {
+    background-color: -libweb-palette-hover-highlight;
+}
+
+:link {
+    color: -libweb-link;
+}
+
+:visited {
+    color: -libweb-palette-visited-link;
+}
+
+:link:active, :visited:active {
+    color: -libweb-palette-active-link;
+}

Userland/Libraries/LibWebView/OutOfProcessWebView.cpp

@@ -436,4 +436,10 @@ void OutOfProcessWebView::set_user_style_sheet(String source)
     client().async_set_user_style(source);
 }
 
+void OutOfProcessWebView::use_native_user_style_sheet()
+{
+    extern StringView native_stylesheet_source;
+    set_user_style_sheet(MUST(String::from_utf8(native_stylesheet_source)));
+}
+
 }

Userland/Libraries/LibWebView/OutOfProcessWebView.h

@@ -55,6 +55,9 @@ public:
     void set_content_scales_to_viewport(bool);
 
     void set_user_style_sheet(String source);
+    // Load Native.css as the User style sheet, which attempts to make WebView content look as close to
+    // native GUI::Widgets as possible.
+    void use_native_user_style_sheet();
 
 private:
     OutOfProcessWebView();