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 */