Add B_SCROLL_BAR_THUMB_COLOR constant.

This allows you to change the scrollbar thumb color in Appearance preferences.
The default color is 216, 216, 216 so the scroll bar thumb looks the same by
default. Perhaps someday this can be updated to something a bit more colorful.

src/kits/interface/ScrollBar.cpp

@@ -965,11 +965,13 @@ BScrollBar::Draw(BRect updateRect)
 			bottomOfThumb, updateRect, normal, flags, fOrientation);
 	}
 
+	rgb_color thumbColor = ui_color(B_SCROLL_BAR_THUMB_COLOR);
+
 	// Draw scroll thumb
 	if (enabled) {
 		// fill the clickable surface of the thumb
 		be_control_look->DrawButtonBackground(this, rect, updateRect,
-			normal, 0, BControlLook::B_ALL_BORDERS, fOrientation);
+			thumbColor, 0, BControlLook::B_ALL_BORDERS, fOrientation);
 		// TODO: Add the other thumb styles - dots and lines
 	} else {
 		if (fMin >= fMax || fProportion >= 1.0 || fProportion < 0.0) {