How can I add more than one Code in the CSS area?

Edit 

You have syntax errors in your code. Most rule-sets only have a single open/close curly bracket.

@media queries are a bit different in that you enclose rule-sets within it.

Here is some cleaned-up code but I haven't tested it.

@media screen and ( max-width : 767px ) {

  .products .list-grid {
  
    grid-template-columns: repeat(2,minmax(0,1fr)) !important;
    display: grid;
    grid-column-gap: 10px;
    grid-row-gap: 20px;
    
    }
    
  }

.products .grid-item { 
  
  width: 48%;
  
  }

.products.collection-content-wrapper .products-flex-container .list-grid .grid-item.sold-out {

  order: 2;
  
  }

.products.collection-content-wrapper .products-flex-container .list-grid {

  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
  
  }

@media only screen and (max-width: 768px) {

  .products.collection-content-wrapper .products-flex-container .list-grid {
    
    grid-template-columns: 1fr;
    
    }
    
  .products.collection-content-wrapper .products-flex-container .list-grid .grid-item {
  
    width: 100%;
    
    }
    
  }

.products.collection-content-wrapper .products-flex-container .list-grid .grid-item {

  width: 80%;
  
  }

Let us know how it goes.