Create white text on a black background

I’ve read a few posts on this but I still don’t understand it. Why this code doesn’t create a white text on a black background:

lv_obj_t *hello_world_label;
lv_obj_set_style_bg_color(lv_scr_act(), lv_color_black(), LV_PART_MAIN);
lv_obj_set_style_text_color(lv_scr_act(), lv_color_white(), LV_PART_MAIN);
hello_world_label = lv_label_create(lv_scr_act());
lv_label_set_text(hello_world_label, "Hello world!");
lv_obj_align(hello_world_label, LV_ALIGN_CENTER, 0, 0);

The result from the simulator. The code is ok