How to change background color of the startup screen

The startup screen background color is always white.
I could not find any info about how to change this color.
I tried to create new Window with the same size as the LCD, and then set its background color to maybe black:
lv_obj_t * win = lv_win_create((lv_obj_t*)lv_scr_act(), (lv_coord_t)(0,0,800,480));
lv_obj_set_style_bg_color(win, (lv_color_t)LV_COLOR_MAKE(0, 0, 0), 0);
But the above code did not change the background color.
Is there any sample code for this ?
Thanks for any help.

No need for an extra object:

lv_obj_set_style_bg_color(lv_scr_act(), LV_COLOR_MAKE(0, 0, 0), LV_STATE_DEFAULT);

That worked.

I have a problem with this command
It does’t cover full screen