.head{
  display: flex; 
  position: sticky;
  top: 0;
  z-index:1;/*最前面固定*/
  background: #3b82c4;/*#3b82c4*/
  }

.nav_menu{
  display: flex; 
  }

.nav_menu_item{
  color: #3b82c4;/*#3b82c4*/
  margin-left: 20px;
  margin-right: 20px;
  }

.nav_url{text-decoration: none;
  color: #ffffff;
}



.container_exhibition {
    display: grid;
    gap: 24px;
    grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
    margin: 6% auto 50px;
}

@media (min-width: 1200px) {
    .container_exhibition {
        grid-template-columns: repeat(3, 1fr);
    }
}

.grid-item {
    text-align: center;
}

.poster {
    width: 100%;
    height: auto;
    vertical-align: top;
    border: 1px solid #ccc;
    box-sizing: border-box;
}