table-cell을 이용한 가로, 세로 중앙 정렬
: http://blog.302chanwoo.com/2016/07/vertical-center/
커스텀 로딩바 만들기
: https://www.w3schools.com/howto/tryit.asp?filename=tryhow_css_loader
flex-wrap
: 줄바꿈 여부
: https://developer.mozilla.org/ko/docs/Web/CSS/flex-wrap
HTML input color
: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/color