diff --git a/src/syntax.c b/src/syntax.c
index 42a0bdc..10805f4 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -7834,18 +7834,25 @@
 			must_redraw = CLEAR;
 			if (color >= 0)
 			{
+			    int dark = -1;
+
 			    if (termcap_active)
 				term_bg_color(color);
 			    if (t_colors < 16)
-				i = (color == 0 || color == 4);
-			    else
-				i = (color < 7 || color == 8);
+				dark = (color == 0 || color == 4);
+			    /* Limit the heuristic to the standard 16 colors */
+			    else if (color < 16)
+				dark = (color < 7 || color == 8);
 			    /* Set the 'background' option if the value is
 			     * wrong. */
-			    if (i != (*p_bg == 'd'))
+			    if (dark != -1
+				    && dark != (*p_bg == 'd')
+				    && !option_was_set((char_u *)"bg"))
+			    {
 				set_option_value((char_u *)"bg", 0L,
-					i ?  (char_u *)"dark"
-					  : (char_u *)"light", 0);
+				       (char_u *)(dark ? "dark" : "light"), 0);
+				reset_option_was_set((char_u *)"bg");
+			    }
 			}
 		    }
 		}