JSFiddle - React, Tailwind, and code Playground

HTML

<div class="knobotri">
 <div>
  Х
 </div>
</div>

CSS

.knobotri{
  position: fixed;
  bottom: 20px;
  right: 20px;
border: 1px solid #2cd4455e;
padding: 30px;
background: #2cd44527;
  height: 70px;
  width: 70px;
  border-radius: 90px;
}
.knobotri div{
  height: 40px;
  width: 40px;
  padding: 10px;
  background: #2CD445;
  color: #fff;
  border-radius: 90px;
  font-size: 30px;
}
.knobotri, .knobotri div{
    display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
    -webkit-box-pack: center;
    -ms-flex-pack: center;
    justify-content: center;
    -webkit-box-align: center;
    -ms-flex-align: center;
    align-items: center;
}