/* You can either copy the below and paste it between style tags on your HTML page, OR you can copy this to a file and name it .css and link it from your HTML page. */
:root {
  --bg-color:#d7f8e5;
  --border-color:#7c9b89;
  --subtitle-color:#35a363;
  --link-color:#18bc5a;
}

html, body {
  margin:0;
  padding:0;
  background-color:#a7efb8;
  background-attachment:fixed;
  background-image:url('backgroundplaceholder.png');
}
body a {
  color:var(--link-color);
}
header {
  width:400px;
  height:400px;
  background-image:url('https://i.imgur.com/lZZP1Md.png');
  background-size:103%;
  
}
.flex {
  display:flex;
}
.sidebar {
  background-color:var(--bg-color);
  max-width:250px;
  width:200px;
  height:100vh;
  border:1px solid var(--border-color);
  position:fixed;
}
article {
  background-color:var(--bg-color);
  border:1px solid var(--border-color);
  margin-top:20px;
  max-width:370px;
  margin-left:10px;
  margin-right:10px;
  
}
.subtitle {
  color:var(--subtitle-color);
  font-size:18px;
  font-weight:bold;
  border-bottom:2px solid var(--subtitle-color);
  margin-left:10px;
  margin-right:10px;
  text-align:right;
  padding-top:20px;
}
article > p {
  padding:10px;
  padding-left:20px;
}
.links {
  list-style-type:'+';
  font-size:16px;
  
}
.links li a {
  color:var(--link-color);
  letter-spacing:1px;
  text-decoration:none;
}
main {
  margin-top:-21px;
}
   section > p {
     padding:10px;
   }
footer {
  margin-left:20px;
  max-width:30%;
  font-size:12px;
}

 @media only screen and (max-width: 630px) {
   .sidebar {
     position:relative;
     max-width:100%;
     width:100%;
     height:150px;
     display:flex;
     flex-direction:row;
     flex-wrap:wrap;
     overflow:hidden;
   }
   .flex {
     flex-wrap:wrap;
     flex-direction:row;
   }
   .right {
     order:1;
     width:100%;
   }
   .left {
     order:2;
   }
   section {
     display:flex;
     flex-wrap:wrap;
     width:120px;
   }
}
<link href="/style.css" rel="stylesheet" type="text/css" media="all">