Make center divider contrast in dark mode

Previously the center climb-rate divider line was always set to black. Update applies darkMode to choose white when in dark mode (otherwise black) so the center line contrasts with the screen background. Also reworded the comment to reflect the contrast behavior.

Author: zjwhitehead

src/sp140/lvgl/lvgl_main_screen.cpp

@@ -586,9 +586,9 @@ void setupMainScreen(bool darkMode) {
 
     lv_line_set_points(climb_rate_divider_lines[i], line_points[i], 2);
 
-    // Make the center line (line 6) special: 3 pixels wide and black
+    // Make the center line (line 6) special: 3 pixels wide; contrasts with screen bg
     if (i == 6) {
-      lv_obj_set_style_line_color(climb_rate_divider_lines[i], LVGL_BLACK, LV_PART_MAIN);
+      lv_obj_set_style_line_color(climb_rate_divider_lines[i], darkMode ? LVGL_WHITE : LVGL_BLACK, LV_PART_MAIN);
       lv_obj_set_style_line_width(climb_rate_divider_lines[i], 3, LV_PART_MAIN);
     } else {
       lv_obj_set_style_line_color(climb_rate_divider_lines[i], LVGL_GRAY, LV_PART_MAIN);