.container {
  display:grid;
  grid-gap: 5px;
  grid-template:
    "header header"
    "marquee marquee"
    "sidebar art-gallery"
    "footer footer"
    / 4fr;
    background-image: url('Clouds.gif');
    background-size: 500px 500px;
    background-repeat: repeat;
    padding:0;
}

header { grid-area: header; }
aside { grid-area: sidebar; }
art-gallery { grid-area: art-gallery; }
footer { grid-area: footer; }


.header{
  height: 300px;
  width: 100%;
  }

.art-gallery div {
  width: 25%;
  margin: 5px;
  border: 5px solid;
}

#double {
  width: 50%;
  margin: 5px;
  border: 5px solid;
}

.art-gallery div img {
  width: 100%;
  height: 100%;
  object-fit: cover;
}

.art-gallery {
  display: flex;
  flex-wrap: wrap;
}

/* Items */
.art-gallery div {
  box-sizing: border-box;
  width: calc(25% - 10px);
}

/* Mobile */
@media (max-width: 800px) {
  .art-gallery div {
    width: calc(50% - 10px);
  }
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
}