Set gba_darken opt default to 37

Set the GBA darken value default to 37 instead of 50, seems to look
pretty good.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

src/wx/config/internal/option-internal.cpp

@@ -192,7 +192,7 @@ std::array<Option, kNbOptions>& Option::All() {
         bool link_proto = false;
 #endif
         wxString gba_rom_dir;
-        uint32_t gba_darken = 50;
+        uint32_t gba_darken = 37;
 
         /// Core
         bool agb_print = false;