Allow usage of a CSS variable for the scroll offset setting

You can now define such a scroll offset setting:

var(--toc-scroll-offset, 1.4rem)

And then tweak the --toc-scroll-offset using media queries in your CSS files, as can ben seen in the example below:

:root {
  --toc-scroll-offset: 8.2rem;
}

@media (min-width: 992px) {
  :root {
    --toc-scroll-offset: 5.4rem;
  }
}

@media (min-width: 1200px) {
  :root {
    --toc-scroll-offset: 1.4rem;
  }
}