How do you customize the boxes in the calander?

How do i customize the little boxes inside a calendar widget? I want to make the date boxes inside a calendar widget get highlighted (the entire box not just the borders) when I click on a date. also I want to remove the grids around the date boxes. I am currently practising on the LVGL practice code for the calendar widget that can be found in this link: Calendar (lv_calendar) — LVGL documentation