CThemes: add default theme from config and migrate theme settings if needed

acinclude.m4

@@ -38,6 +38,13 @@ AC_ARG_ENABLE(reschange,
 	AC_DEFINE(ENABLE_CHANGE_OSD_RESOLUTION, 1, [enable change the osd resolution]))
 AM_CONDITIONAL(ENABLE_RESCHANGE, test "$enable_reschange" = "yes")
 
+# default theme
+AC_ARG_WITH(default-theme,
+	AS_HELP_STRING([--with-default-theme=THEMENAME], [Default theme for gui. Default it is empty for internal fallback to default colors.]),
+	[default_theme="$withval"],
+	[default_theme=""])
+AC_DEFINE_UNQUOTED([DEFAULT_THEME], ["$default_theme"], [Default theme for gui.])
+
 AC_MSG_CHECKING(target)
 
 if test "$TARGET" = "native"; then