style.css

:root {
  --primary-color: #4dba87;
  --lightGrey-color: #d2d6dd;
  --darkGrey-color: #2c2c2c;
  --font-family: -apple-system, BlinkMacSystemFont, Avenir, "Avenir Next", "Segoe UI", "Roboto", "Oxygen", "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue", sans-serif;
  --font-weight: 400;
  --font-size: 1.6rem;
  --line-height: 1.6;
}

html,
body {
  width: 100%;
  height: 100%;
}

html {
  font-size: 62.5%;
  -webkit-box-sizing: border-box;
          box-sizing: border-box;
  -ms-text-size-adjust: 100%;
  -webkit-text-size-adjust: 100%;
  -ms-overflow-style: scrollbar;
  -webkit-tap-highlight-color: rgba(0, 0, 0, 0);
}

*, *:before, *:after {
  -webkit-box-sizing: inherit;
          box-sizing: inherit;
  margin: 0;
  padding: 0;
}

body {
  line-height: var(--line-height);
  font-size: var(--font-size);
  font-weight: var(--font-weight);
  font-family: var(--font-family);
  color: var(--darkGrey-color);
}

.preview {
  -webkit-box-flex: 1;
      -ms-flex: 1 0 auto;
          flex: 1 0 auto;
  position: relative;
  display: block;
  background-color: var(--darkGrey-color);
  width: 100%;
  height: 100%;
}

.preview__iframe {
  position: absolute;
  border: none;
  width: 100%;
  height: calc(100% - 60px);
  margin-top: 60px;
}

.preview__iframe--tablet,
.preview__iframe--mobile {
  left: 50%;
  -webkit-transform: translateX(-50%);
          transform: translateX(-50%);
}

.preview__iframe--tablet {
  width: 768px;
}

.preview__iframe--mobile {
  width: 375px;
}

.navbar {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -ms-flex-wrap: wrap;
      flex-wrap: wrap;
  -webkit-box-align: center;
      -ms-flex-align: center;
          align-items: center;
  -webkit-box-pack: justify;
      -ms-flex-pack: justify;
          justify-content: space-between;
  min-height: 60px;
  background: #fefafc;
  position: fixed;
  top: 0;
  right: 0;
  left: 0;
  z-index: 1030;
  border-bottom: 4px solid var(--primary-color);
}

.navbar__container {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -ms-flex-wrap: wrap;
      flex-wrap: wrap;
  -webkit-box-align: center;
      -ms-flex-align: center;
          align-items: center;
  -webkit-box-pack: justify;
      -ms-flex-pack: justify;
          justify-content: space-between;
  padding-right: 1rem;
  padding-left: 1rem;
}

.navbar__prev {
  line-height: 55px;
  font-size: 12px;
  text-decoration: none;
  color: var(--primary-color);
  display: inline-block;
}

.navbar__prev svg,
.navbar__prev span {
  display: inline-block;
  vertical-align: middle;
  padding: 0;
  margin: 0;
}

.navbar__nav {
  list-style: none;
  position: absolute;
  left: 50%;
  top: 50%;
  -webkit-transform: translate(-50%, -50%);
          transform: translate(-50%, -50%);
}

.navbar__item {
  display: inline-block;
  position: relative;
  padding: 0 .2rem;
}

.navbar__link {
  display: inline-block;
  line-height: 60px;
  padding: 0 2rem;
}

.navbar__link svg {
  display: inline-block;
  padding: 0;
  margin: 0;
  vertical-align: middle;
  stroke: var(--darkGrey-color);
}

.navbar__link--active {
  background-color: rgba(0, 0, 0, .05);
}

.navbar__link--active svg {
  stroke: var(--primary-color);
}

.navbar__link--active:before {
  content: '';
  width: 0;
  height: 0;
  border-style: solid;
  border-width: 0px 10px 9px 10px;
  border-color: transparent transparent var(--primary-color) transparent;
  position: absolute;
  left: 50%;
  -webkit-transform: translateX(-50%);
          transform: translateX(-50%);
  bottom: -1px;
}