color: use bright yellow to highlight a search match

Plain yellow is too dark (more like brown) on some machines.
Lightyellow is a bit loud, but... very visible.

doc/nano.texi

@@ -894,7 +894,7 @@ text.  (The color of the stripe can be changed with @code{set stripecolor}.)
 
 @item set highlightcolor [bold,][italic,]@var{fgcolor},@var{bgcolor}
 Use this color combination for highlighting a search match.
-The default value is @t{black,yellow}.
+The default value is @t{black,lightyellow}.
 @xref{@code{set functioncolor}} for valid color names.
 
 @item set historylog

doc/nanorc.5

@@ -148,7 +148,7 @@ text.  (The color of the stripe can be changed with \fBset stripecolor\fR.)
 .TP
 .B set highlightcolor \fR[\fBbold,\fR][\fBitalic,\fR]\fIfgcolor\fB,\fIbgcolor\fR
 Use this color combination for highlighting a search match.
-The default value is \fBblack,yellow\fR.
+The default value is \fBblack,lightyellow\fR.
 See \fBset titlecolor\fR for valid color names.
 .TP
 .B set historylog

src/color.c

@@ -59,7 +59,7 @@ void set_interface_colorpairs(void)
 			else if (index == GUIDE_STRIPE)
 				interface_color_pair[index] = A_REVERSE;
 			else if (index == HIGHLIGHTED) {
-				init_pair(index + 1, COLOR_BLACK, COLOR_YELLOW);
+				init_pair(index + 1, COLOR_BLACK, COLOR_YELLOW + (COLORS > 15 ? 8 : 0));
 				interface_color_pair[index] = COLOR_PAIR(index + 1);
 			} else if (index == PROMPT_BAR)
 				interface_color_pair[index] = interface_color_pair[TITLE_BAR];