/* base */
:root {
  height: 100%;
  width: 100%;
  box-sizing: border-box;
}
*,
::before,
::after {
  box-sizing: inherit;
}
body {
  position: relative;
  height: 100%;
  width: 100%;
  font-family: $futura;
  background-color: $gray;
}

/* navigation */
.l-admin-nav {
  width: 4.06em;
  position: absolute;
  left: 0;
  top: 4.06em;
  bottom: 0;
  background-color: $green;
}
.l-admin-nav__list {
  padding-left: 0;
  list-style: none;
}
.l-admin-nav__list > li:nth-of-type(1) {
  margin-top: 3em;
  margin-bottom: 5.79em;
}
.l-admin-nav__item {
  padding-top: 0.25em;
  padding-bottom: 0.25em;
  border-left: 2px solid $green;
}

<nav class="l-admin-nav">
  <ul class="l-admin-nav__list">
    <li class="l-admin-nav__item"><%= image_tag "asdf.png" %></li>
    <li class="l-admin-nav__item"><%= image_tag "asdf.png" %></li>
    <li class="l-admin-nav__item"><%= image_tag "asdf.png" %></li>
    <li class="l-admin-nav__item"><%= image_tag "asdf.png" %></li>
  <ul>
</nav>