css button border

.placeholder-box {
  position: relative;
  display: block;
  width: 300px;
  min-height: 150px;
  margin-top: 50px;
  padding: 30px;
  border: 1px solid #888;
  border-radius: 10px;
  box-sizing: border-box;
  box-shadow: 0 0 8px 0 rgba(0,0,0,0.25);
}

.placeholder-box p {
  text-align: center;
  margin: 0 0 15px;
}

.placeholder-box ul {
  margin: 0;
}

/* Button */

button {
 display: block;
 border: solid white;
 border-width: 0 10px;
 border-radius: 0;
 background: green;
 color: white;
 font-size: 18px;
 margin: -60px auto 15px;
 padding: 15px;
}