LV_PART_ITEMS → Keyboard buttons (individual keys)
Then combine with LV_STATE_PRESSED
https://docs.lvgl.io/master/widgets/keyboard.html
lv_obj_set_style_text_color(kb, lv_color_black(), LV_PART_ITEMS | LV_STATE_PRESSED);
lv_obj_set_style_background_color(kb, lv_color_white(), LV_PART_ITEMS | LV_STATE_PRESSED);