<ul>
  <li class="a">Put on!</li>
</ul>

li {
    display : inline-block;
    background: #74653D; 
    color: #FFFBD5; 
    width: 300px;
    line-height: 66px;
    text-align: center;
    font-size: 28px;
    font-weight: bold;
    -webkit-transition: all 0.5s;
    transition: all 0.5s;
}
.a:hover {
  border-radius: 33px; /*角が丸くなります*/
}