Add dark mode toggle

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

config.def.h

@@ -20,6 +20,7 @@ static Parameter defconfig[ParameterLast] = {
 	[Certificate]         =       { { .i = 0 },     },
 	[CaretBrowsing]       =       { { .i = 0 },     },
 	[CookiePolicies]      =       { { .v = "@Aa" }, },
+	[DarkMode]            =       { { .i = 0 },     },
 	[DefaultCharset]      =       { { .v = "UTF-8" }, },
 	[DiskCache]           =       { { .i = 1 },     },
 	[DNSPrefetch]         =       { { .i = 0 },     },
@@ -179,6 +180,7 @@ static Key keys[] = {
 	{ MODKEY|GDK_SHIFT_MASK, GDK_KEY_b,      toggle,     { .i = ScrollBars } },
 	{ MODKEY|GDK_SHIFT_MASK, GDK_KEY_t,      toggle,     { .i = StrictTLS } },
 	{ MODKEY|GDK_SHIFT_MASK, GDK_KEY_m,      toggle,     { .i = Style } },
+	{ MODKEY|GDK_SHIFT_MASK, GDK_KEY_d,      toggle,     { .i = DarkMode } },
 };
 
 /* button definitions */

surf.1

@@ -208,6 +208,9 @@ Toggle scrollbars. This will reload the page.
 .B Ctrl\-Shift\-c
 Toggle caret browsing. This will reload the page.
 .TP
+.B Ctrl\-Shift\-d
+Toggle dark mode. This will reload the page.
+.TP
 .B Ctrl\-Shift\-i
 Toggle auto-loading of images. This will reload the page.
 .TP

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 ?