discourse/app/assets/stylesheets/common/base/activation.scss

// Styles used before the user is logged into discourse. For example, activating their
// account or changing their email.

#simple-container {
  border-radius: 10px;
  background-color: var(--secondary);
  padding: 20px;
  width: 550px;
  margin: 0 auto;

  .account-created {
    .ac-message {
      font-size: $font-up-1;
      line-height: $line-height-large;
    }
    .activation-controls {
      button {
        margin-right: 0.5em;
      }
    }
    .ac-page {
      border-radius: 10px;
      margin-top: 25px;
      box-shadow: 0 4px 14px rgba(0, 0, 0, 0.15);
      border-top: 6px solid var(--tertiary);
    }
  }
}