圖片居中

#quiz_start_panel {

background-image: url(/dom/assets/exercise_start.png);

width: 40vw;

height: 21vw;

background-size: 40vw 21vw;

position: absolute;

top: 50%;

left: 50%;

margin-left: -20vw;

margin-top: -11vw;

}