마진(margin)

1: 0.25rem, 2: 0.5rem, 3: 1rem, 4: 1.5rem, 5: 3rem

m: margin-top
mt: margin-top
me: margin-right
mb: margin-bottom
ms: margin-left
mx: X축 margin
my: Y축 margin
mx-auto: 박스 중앙 정렬