patch 8.1.2034: dark them of GTK 3 not supported

Problem:    Dark them of GTK 3 not supported.
Solution:   Add the "d" flag in 'guioptions'. (Jonathan Conder, closes #4934)
diff --git a/src/option.h b/src/option.h
index bb68d5a..1d85843 100644
--- a/src/option.h
+++ b/src/option.h
@@ -213,6 +213,7 @@
 #define GO_ASELML	'A'		// autoselect modeless selection
 #define GO_BOT		'b'		// use bottom scrollbar
 #define GO_CONDIALOG	'c'		// use console dialog
+#define GO_DARKTHEME	'd'		// use dark theme variant
 #define GO_TABLINE	'e'		// may show tabline
 #define GO_FORG		'f'		// start GUI in foreground
 #define GO_GREY		'g'		// use grey menu items
@@ -231,7 +232,7 @@
 #define GO_FOOTER	'F'		// add footer
 #define GO_VERTICAL	'v'		// arrange dialog buttons vertically
 #define GO_KEEPWINSIZE	'k'		// keep GUI window size
-#define GO_ALL		"!aAbcefFghilmMprtTvk" // all possible flags for 'go'
+#define GO_ALL		"!aAbcdefFghilmMprtTvk" // all possible flags for 'go'
 
 // flags for 'comments' option
 #define COM_NEST	'n'		// comments strings nest