How to style a dropdown using CSS code example

Example 1: how to add a selection in css

.custom-select-wrapper {
     position: relative;
     user-select: none;
     width: 100%;
}
 .custom-select {
     position: relative;
     display: flex;
     flex-direction: column;
     border-width: 0 2px 0 2px;
     border-style: solid;
     border-color: #394a6d;
}
 .custom-select__trigger {
     position: relative;
     display: flex;
     align-items: center;
     justify-content: space-between;
     padding: 0 22px;
     font-size: 20px;
     font-weight: 300;
     color: #3b3b3b;
     height: 60px;
     line-height: 60px;
     background: #ffffff;
     cursor: pointer;
     border-width: 2px 0 2px 0;
     border-style: solid;
     border-color: #394a6d;
}
 .custom-options {
     position: absolute;
     display: block;
     top: 100%;
     left: 0;
     right: 0;
     border: 2px solid #394a6d;
     border-top: 0;
     background: #fff;
     transition: all 0.5s;
     opacity: 0;
     visibility: hidden;
     pointer-events: none;
     z-index: 2;
}
 .custom-select.open .custom-options {
     opacity: 1;
     visibility: visible;
     pointer-events: all;
}
 .custom-option {
     position: relative;
     display: block;
     padding: 0 22px 0 22px;
     font-size: 22px;
     font-weight: 300;
     color: #3b3b3b;
     line-height: 60px;
     cursor: pointer;
     transition: all 0.5s;
}
 .custom-option:hover {
     cursor: pointer;
     background-color: #b2b2b2;
}
 .custom-option.selected {
     color: #ffffff;
     background-color: #305c91;
}

Example 2: how to add a selection in css

*, *:after, *:before {
     box-sizing: border-box;
}