test.css

@media screen and (min-width: 481px){ 
	body{
		margin: 0px 350px;
		width: 1280px;
		background-color:#fff;
		font-family:"Yu Gothic Medium";
		}
	header{
		margin:30px;
		height:80px;
	}
	header ul{
	  list-style: none;
	  position: relative;
	  top: 10px;
	  left: 100px;
	  font-size:30px;
	}
	header li{
		display: inline-block;
		margin-left: 20px;

	}
	header .container{
		display: flex;
	}
	header h1{
		font-size: 50px;
		color:#000;
	}
	.main{
		height: 600px;
		padding: 0px 100px;
	}

	.contents img{
		margin-left:5px;
		margin-top: 5px;
		vertical-align: bottom
	}
	.contents img:hover{
		opacity: .5;

	}
	footer{
		color:#000;
		font-size:30px;
		margin:30px;
	}
	#twiiterURL{
		text-decoration: none;
	}
}
@media screen and (max-width: 480px){
	header{
		margin:30px;
		height:40px;
		color: #000;
	}
	header .container{
		display: flex;
	}
	
	header ul{
	  list-style: none;
	  position: relative;
	  top: 10px;
	  left: 10px;
	}
	header li{
		display: inline-block;

	}
	.main .container{
		display: flex;
  		flex-direction: column;
  		align-items: center;
	}
	.contents img{
		margin-left:5px;
		margin-top: 5px;
	}
	footer{
		color:#000;
		margin:30px;
	}
 }