style.css

/* GREY BACKGROUND WITH NO MARGIN */
body{
    background: darkgrey;
    height: 2000px;
    margin: 0;
}

a{
    text-decoration: none;
    font: bold 18px 'segoe ui';
    color: grey
}
  
  /* WHITE HEADER */
header{
    background: white;
    height: 70px;
    margin: auto;
}
  
/* #DROPDOWNMENU IS THE MAIN BLOCK WHICH INCLUDE :
- HIDDEN CHECKBOX (OPACITY 0%)
- VISIBLE TOGGLE BUTTON
- VISIBLE BUTTON ANIMATION (TOP, MIDDLE, BOTTOM BAR)
- LIST BLOCK INCLUDING LIST ELEMENT*/
#dropdownmenu{
    position: fixed;
    width: 150px;
    right: 20px;
    top: 20px;
    height: 36px;
}
  
#dropdownmenu .list{
    position: relative;
    margin: auto;
    width: auto;
    background: none;
    top: -30px;
    display: flex;
    flex-direction: column;
    gap: 3px;
}

.list_element{
    display: flex;
    position: relative;
    justify-content: center;
    align-items: center;
    background: white;
    width: 150px;
    border-radius: 5px;
    height: 35px;
    box-shadow: 0px 0px 2px grey;
    transition-duration: 2s;
}
  
.list_element:hover, .list_element:active{
    transform: none;
    background: #f2bc52;
    color: white;
    transition-duration: .2s;
}

  /* VISIBLE BUTTON AND CHECKBOX SHARE THE SAME SIZE AND POSITION
  SET CHECKBOX OPACITY TO 1 TO SEE IT */
#visible_button{
    display: flex;
    flex-wrap: nowrap;
    flex-direction: column;
    justify-content: space-between;
    position: relative;
    top: -43px;
    left: 112px;
    height: 26px;
    width: 28px;
    z-index: 10;
    background: rgba(255, 255, 255, 0.7);
    padding: 5px;
    border-radius: 4px;
}
  
#checkbox {
    width: 38px;
    height: 36px;
    z-index: 9999;
    position: relative;
    left: 108px;
    top: -3px;
    opacity: 0;
    cursor: pointer; /* SET POINTER TO GET THE STANDARD LINK HOVER */
}
  
/* ELEMENTS OF ANIMATION */
#top_bar{
    position:relative;
    height: 4px;
    width: 28px;
    border-radius: 2px;
    background: #f2bc52;
    box-shadow: 1px 1px 2px grey;
    transition-duration: 1s;
}

#middle_bar{
    height: 4px;
    width: 28px;
    border-radius: 2px;
    background: #f2bc52;
    box-shadow: 1px 1px 2px grey;
    transition-duration: .5s;
}
  
#bottom_bar{
    position:relative;
    height: 4px;
    width: 28px;
    border-radius: 2px;
    background: #f2bc52;
    box-shadow: 1px 1px 2px grey;
    transition-duration: 1s;
}
  
/* ANIMATION */
  
#checkbox:not(:checked) ~ #visible_button #top_bar{
    transform: rotate(45deg) translate(7.5px, 7.5px);
    background: grey;
    transition-delay: .5s;
    box-shadow: 1.5px 0.25px 2px grey;
}
  
#checkbox:not(:checked) ~ #visible_button #bottom_bar{
    transform: rotate(-45deg) translate(8px, -8px);
    background: grey;
    transition-delay: .5s;
    box-shadow: -1px 1px 2px grey;
}
  
#checkbox:not(:checked) ~ #visible_button #middle_bar{
    opacity: 0;
}
  
#checkbox:checked ~ #middle_bar{
    transition-delay: 1s;
}
  
/* HIDE LIST BLOCK WHEN CHECKBOX IS CHECKED AND VICE VERSA */
  
#checkbox:checked ~ .list{
    visibility: hidden;
    opacity: 0;
    transition-duration: .5s;
}

#checkbox:not(:checked) ~ .list{
    visibility: visible;
    transition-duration: .5s;
}