.full-size-bg {
  position: relative;
}

.full-size-bg::before {
  --cont-w: 100%;
  --off: calc((100vw - var(--cont-w)) / 2);
  content: "";
  position: absolute;
  left: calc(-1 * var(--off));
  top: 0;
  z-index: -1;
  width: calc(var(--off) * 2 + var(--cont-w));
  height: 100%;
  background: var(--block-bg);
  background-position: center;
  background-size: cover;
  background-repeat: no-repeat;
}

@media (max-width: 768px) {
  .full-size-bg::before {
    left: -14px;
    width: calc(100% + 28px);
  }
}