<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.8.2/css/all.css" integrity="sha384-oS3vJWv+0UjzBfQzYUhtDYW+Pj2yciDJxpsK1OYPAYjqT085Qq/1cq5FLXAZQ7Ay" crossorigin="anonymous">
<link rel="stylesheet/less" href="css/styles.less">
<link rel="stylesheet" href="css/styles.css">
<link rel="stylesheet" href="css/media.css" media="only screen and (max-width:840px)">
<link href="https://fonts.googleapis.com/css?family=Roboto:300,400,700&display=swap" rel="stylesheet"> <!--roboto-->
<title>
Boots4 project
</title>
</head>
<body>
@media only screen and (max-width:1450px){
nav a{
font-size: 18px;
margin-left:20px;
}
} /*1450px*/
@media only screen and (max-width:1010px){
nav a{
margin-left:15px;
}
.search{
display: none;
}
} /*1010px*/
@media only screen and (max-width:840px){
.logo{
font-size: 18px;
}
nav a{
margin-left:10px;
font-size: 15px;
}
.header-main{
.title{
font-size: 50px;
}
.full-btn{
margin-top:20px;
width: 150px;
height: 50px;
font-size: 15px;
}
.border-btn{
margin-top:20px;
width: 150px;
height: 50px;
font-size: 15px;
}
}
} /*840px*/
@media only screen and (max-width:490px){
.header-main{
.full-btn{
display: block;
width: 90%;
}
.border-btn{
display: block;
width: 90%;
}
}
} /*490px*/
<link rel="stylesheet" href="css/media.css" media="only screen and (max-width:840px)">
@media only screen and (max-width:840px){
.....
.header-main{
.title{
font-size: 50px;
}
....
...
}