body {
    max-width: 800px;
    width: 90%;
    margin-left: auto;
    margin-right: auto;
    align-content: center;
}
@media screen and (min-width:480px){
    #stretchable-grid-items {
    width: 50%;
    }
  }

@media screen and (max-width:480px){
    #stretchable-grid-items {
    width: 100%;
    }
  }

#fixed-size-grid-items ul {
    padding-left: 0;
    list-style: none;
    display: flex;
    flex-wrap: wrap;
    }
aside {
    width: 170px;
    margin: 10px;
    font-size: .8rem;
}
#stretchable-grid-items ul {
    padding-left: 0;
    list-style: none;
    display: flex;
    flex-wrap: wrap;
    margin-left: -10px;
    margin-right: -10px;
    margin-top: 1rem;
    margin-bottom: 1rem;
  }
  #stretchable-grid-items li {
    width: 100%;
  }