<div class="box">
  <p class="box-default">
    I'm fatfish, 6 years of programming experience, like front-end, writing
    and making friends, looking forward to becoming good friends with you.
  </p>
  <p class="box-custom">
    I'm fatfish, 6 years of programming experience, like front-end, writing
    and making friends, looking forward to becoming good friends with you.
  </p>
</div>

* {
  margin: 0;
  padding: 0; 
}

body {
  padding: 15px;
  color: #324b64;
}

.box > p {
  padding: 10px;
  background-color: #f5f6f9;
  border-radius: 6px;
  font-size: 12px;

  margin-bottom: 15px;
}
/* Key Style */
.box-custom::selection {
  color: #ffffff;
  background-color: #ff4c9f;
}