如何使用align-items HTML+CSS 居中

/* var-css */

:root {
  --main-color: #757575;
  --sub-color: #212121;
  --accent-color: #2196f3;
  --white: #FFFFFF;
  --white-transp: rgba(255, 255, 255, 0.6);
}


/* general */

body {
  font-family: Roboto, sans-serif;
  color: var(--main-color);
}

.list {
  list-style: none;
}

.ref {
  text-decoration: none;
}

.container {
  margin-right: auto;
  margin-left: auto;
  padding-right: 15px;
  padding-left: 15px;
  width: 1200px;
  outline: 1px solid tomato;
}


/* header */

.container.header {
  display: flex;
  align-items: center;
}

.site-nav {
  display: flex;
}

.logo {
  margin-right: 93px;
  font-family: "Raleway", sans-serif;
  font-weight: 700;
  font-size: 26px;
  letter-spacing: 0.03em;
  color: var(--accent-color);
}

.logo .black {
  color: var(--sub-color);
}

.nav-list {
  display: flex;
  margin-top: 0;
  margin-bottom: 0;
  padding-top: 32px;
  padding-bottom: 32px;
  padding-left: 0;
}

.nav-item:not(:last-child) {
  margin-right: 50px;
}

.nav-ref {
  font-weight: 500;
  font-size: 14px;
  letter-spacing: 0.02em;
  color: #212121;
}

.nav-ref:hover,
.nav-ref:focus {
  color: var(--accent-color);
}

.contacts-header {
  display: flex;
  margin-top: 0;
  margin-bottom: 0;
  margin-left: auto;
  padding-left: 0;
}

.contacts-item:not(:last-child) {
  margin-right: 50px;
}

.contacts-ref {
  font-weight: 500;
  font-size: 14px;
  letter-spacing: 0.02em;
  color: #757575;
}

.contacts-ref:hover,
.contacts-ref:focus {
  color: var(--accent-color);
}


/* ----------------------------------------------- */

<header class="header">
  <div class="container header">
    <nav class="site-nav">
      <a href="./index.html" class="logo ref">Web<span class="black">Studio</span>
          </a>

      <ul class="nav-list list">
        <li class="nav-item"><a href="" class="nav-ref ref">Студія</a></li>
        <li class="nav-item">
          <a href="" class="nav-ref ref">Портфоліо</a>
        </li>
        <li class="nav-item">
          <a href="" class="nav-ref ref">Контакти</a>
        </li>
      </ul>
    </nav>

    <ul class="contacts-header list">
      <li class="contacts-item">
        <a href="mailto:[email protected]" class="contacts-ref ref">[email protected]</a
            >
          </li>
          <li class="contacts-item">
            <a href="tel:380961111111" class="contacts-ref ref"
              >+38 096 111 11 11</a
            >
          </li>
        </ul>
      </div>
    </header>