<div class="parent" style="padding-left:30px; padding-right: 30px">
  <div class="parent__child pl_inherit pr_inherit">123</div>
</div>

:root {
  --inherit: inherit;
}

.pl_inherit {
  padding-left: var(--inherit);
}

.pr_inherit {
  padding-right: inherit;
}

.parent {
  background-color:red;
}

.parent__child {
  background-color: green;
}