Add dark mode toggle

This sets dark gtk theme variant,
which should make webkit2gtk use dark css variant
from websites that provide it.

surf.c

@@ -56,6 +56,7 @@ typedef enum {
 	CaretBrowsing,
 	Certificate,
 	CookiePolicies,
+	DarkMode,
 	DiskCache,
 	DefaultCharset,
 	DNSPrefetch,
@@ -259,6 +260,7 @@ char *argv0;
 static ParamName loadtransient[] = {
 	Certificate,
 	CookiePolicies,
+	DarkMode,
 	DiskCache,
 	DNSPrefetch,
 	FileURLsCrossAccess,
@@ -772,6 +774,10 @@ setparameter(Client *c, int refresh, ParamName p, const Arg *a)
 		    cookiepolicy_get());
 		refresh = 0;
 		break;
+	case DarkMode:
+		g_object_set(gtk_settings_get_default(),
+		             "gtk-application-prefer-dark-theme", a->i, NULL);
+		break;
 	case DiskCache:
 		webkit_web_context_set_cache_model(
 		    webkit_web_view_get_context(c->view), a->i ?