宽度小于768px
@media screen and (max-width:768px){
}
宽度在768~992之间
@media screen and (min-width:768px) and (max-width:992px){
}
宽度在992~1200之间
@media screen and (min-width:992px) and (max-width:1200px){
}
宽度在1200以上
@media screen and (min-width:1200px){
}
