.tab {
  overflow: hidden;
  border: none;
  background-color:;
}

/* Style the buttons that are used to open the tab content */
.tab button{
  background-color: inherit;
  float: left;
  border: none;
  outline: none;
  cursor: pointer;
  padding: 14px 16px;
  transition: 0.3s;
}
.tab a{
    background-clip:border-box;
    background-color: inherit;
    float: left;
    border: none;
    outline: none;
    cursor: pointer;
    padding: 12.5px 13px;
    transition: 0.3s;
    text-decoration:none;
    font-family:Arial;
}
/* Change background color of buttons on hover */
.tab button:hover {
  background-color: #ddd;
}

/* Create an active/current tablink class */
.tab button.active {
  background-color: #ccc;
}
.tab button.extension{
    background-color:#DCDCDC
}

/* Style the tab content */
.tabcontent {
  display: none;
  padding: 6px 12px;
  border: none;
  border-top: none;
}
Example
.tabcontent {
  animation: fadeEffect 1s; /* Fading effect takes 1 second */
}

/* Go from zero to full opacity */
@keyframes fadeEffect {
  from {opacity: 0;}
  to {opacity: 1;}
}
.extensions{
    background-color:#DCDCDC;
}
.extensions2 {
  overflow: hidden;
  border: 0px solid #ccc;
  background-color:;
}

/* Style the buttons that are used to open the tab content */
.extensions2 {
  background-color:#f2f2f2;
  border: none;
  outline: none;
  cursor: pointer;
  padding: 1px 1px;
  transition: 0.3s;
  font-size:6px;
  height:20px;
  width:20%;
}
.extensions button:hover{
    background-color:red;
}
#newtabs{
    position:absolute;
    top:69%;
    left:55%;
}