html {
 background-image: url(catcatcat.jpg);
 background-size: 33.4vw 100vh;
}

body {
  background-color: white;
  background-image: url(bg.png);
  width: 1000px;
  height: 500px;
  margin: auto;
  margin-top: 50px;
  color: black;
  font-family: Verdana;
  border-radius: 5px;
  box-shadow: inset 0 0 0 3px black;
}

.header {
  background-color: yellow; 
  font-size: 16px;
  height: 29px;
  border-top-right-radius: 5px;
  border-top-left-radius: 5px;
  border: 3px black solid;
}

.left-tab, .tab {
  height: 24px;
  display: inline-block;
  font-size: 16px;
  padding: 4px;
}

.tab {
  padding: 4px;
  border-left: 3px black solid;
}

h1 {
  text-align: center; 
}

p, ul, ol, button, img {
  margin: 16px 10px; 
}

a {
  color: aqua;
}