<div class="app">
  <header>
    Basic Application
  </header>
  <main>
    <!-- Left sidebar -->
    <aside>
      Sidebar
    </aside>

    <!-- Main content -->
    <article>
      <p>
       Why did the developer go broke? Because he used up all his cache
      </p>

    </article>

  </main>
</div>

:root {
  /*   Background Colors */
  --headerBg: #001b38;
  --sidebarBg: #ba324f;
  --articleBg: #f2f2f4;
  /*   Text Colors */
  --articleTextColor: #999999;
  --headerTextColor: #9aa1b5;
  --sidebarTextColor: #f9aaaa;
  --toggleButtonBackground: #333;
  --toggleButtonColor: #fff;
}
@media (prefers-color-scheme: dark) {
  :root {
    /*   Background Colors */
    --headerBg: #525252;
    --sidebarBg: #414141;
    --articleBg: #313131;
    /*   Text Colors */
    --articleTextColor: rgba(255, 255, 255, 0.3);
    --headerTextColor: rgba(255, 255, 255, 0.8);
    --sidebarTextColor: rgba(255, 255, 255, 0.5);
    --toggleButtonBackground: #fff;
    --toggleButtonColor: #000;
  }
}

/* The properties and declarations below will remain the same */

.app {
  display: flex;
  flex-direction: column;
  height: 100vh;
}
header {
  background: var(--headerBg);
  padding: 10px;
  color: var(--headerTextColor);
  font-weight: bold;
}
main {
  flex-grow: 1;
  display: flex;
  flex-direction: row;
}
aside {
  min-width: 150px;
  background: var(--sidebarBg);
  color: var(--sidebarTextColor);
  padding: 20px 10px;
  font-weight: bold;
}
article {
  flex-grow: 1;
  background: var(--articleBg);
  padding: 10px 20px;
  font-size: 28px;
  color: var(--articleTextColor);
}