discourse/app/assets/stylesheets/desktop/admin_customize.scss

.admin-customize {
  .show-current-style {
    padding-left: 10px;
    padding-top: 10px;
    width: 70%;
  }

  .themes-list {
    width: 28%;
  }

  .customize-themes-header {
    flex-direction: row;
    align-items: center;
  }
}