style.css

/*!
Theme Name: _s-child-theme
Theme URI: https://github.com/jitendersinghwp/_s-child-theme
Author: jitendersinghwp
Author URI: https://profiles.wordpress.org/jitendersinghwp
Template:     _s
Description: Hi. I'm a <code>child-theme</code> of _s.
Version: 1.0.0
License: GNU General Public License v2 or later
License URI: LICENSE
Text Domain: _s-child-theme
Tags: custom-background, custom-logo, custom-menu, featured-images, threaded-comments, translation-ready

This theme, like WordPress, is licensed under the GPL.
Use it to make something cool, have fun, and share what you've learned with others.

_s-child-theme is based on Underscores https://underscores.me/, (C) 2012-2017 Automattic, Inc.
Underscores is distributed under the terms of the GNU GPL v2 or later.

*/

.task{
  display: flex;
  justify-content: center;
  align-items: center;
  flex-direction: column;
  margin-bottom: 20px;
  border:    3px solid black;
  border-radius:    10px;
}
.task.completed{
  background-color: #2ecc71;
}
.task.incompleted{
  background-color: #e74c3c;
}