<div class="modal z-10">
      <div class="modal__body">
        <h2 class="modal__title">Modal Title</h2>
        <button class="modal__close-button">x</button>
      </div>
    </div>
    <header>
      <h1 class="header__title">Labzindex</h1>
      <button class="header__call-to-action z-0">call us</button>
    </header>
    <div class="posts-wraper z-0">
      <div class="posts">
        <article class="posts__item">Post 1</article>
        <article class="posts__item">Post 2</article>
        <article class="posts__item">Post 3</article>
        <article class="posts__item">Post 4</article>
      </div>
    </div>

html {
  --red: #bc442f;
  --green: #007b7d;
  --green: #007b7d;
  --yellow: #ffc978;
}

body {
  background: var(--yellow);
}
h1 {
  margin: 0;
  color: var(--yellow);
}
body,
html {
  padding: 0;
  margin: 0;
}

header {
  background: var(--red);
  padding: 16px;
  text-align: center;
  display: flex;
  align-items: center;
  justify-content: space-around;
  h1 {
    color: var(--yellow);
  }
}

.header__call-to-action {
  z-index: 2;
  border: none;
  text-transform: uppercase;
  padding: 12px;
  border-radius: 4px;
  color: #f2f2f6;
  font-weight: bold;
  background: var(--green);
}

.modal {
  position: absolute;
  width: 100%;
  height: 100%;
  top: 0;
  left: 0;
  z-index: 1;
  display: flex;
  align-items: center;
  justify-content: center;
  background: hsla(0, 0%, 0%, 0.42);
}

.modal__body {
  position: relative;
  color: var(--yellow);
  background: var(--green);
  height: 90%;
  max-width: 500px;
  flex: 1;
  margin: 0 auto;
  padding: 16px;
  box-sizing: border-box;
}

.modal__close-button {
  position: absolute;
  top: -20px;
  right: -20px;
  width: 50px;
  border: none;
  height: 50px;
  font-size: 24px;
  border-radius: 100%;
}

.modal__title {
  font-size: calc(36 / 16 * 1rem);
  margin: 0;
  top: 0;
  left: 16px;
}

.posts {
  display: grid;
  max-width: 600px;
  grid-template-columns: repeat(auto-fill, minmax(min(150px, 100%), 1fr));
  gap: 20px;
  margin: 30px auto;
  position: relative;
  z-index: 1;

  &__item {
    background: white;
    font-size: calc(24 / 16 * 1rem);
    height: 150px;
    border-radius: 6px;
    display: flex;
    align-items: center;
    justify-content: center;
  }
}
$zIndex: (
  "0": 0,
  "10": 10,
  "20": 20,
  "30": 30,
  "40": 40,
  "50": 50,
  "60": 60,
  "70": 70,
  "80": 80,
  "90": 90,
  "100": 100,
  "auto": "auto",
);

@each $index, $level in $zIndex {
  .z-#{$index} {
    z-index: $level;
  }
}