assuming you can edit the html:
div.single-column div.middle {
padding: 10px 20px;
clear: both;
position: relative;
top: 0; // default
}
div.single-column div.middle--alt {
top: 70px;
} // modifier, a la BEM
in your markup on the all pages:
<div class="middle">...</div>
and on the page(s) that need top: 70px:
<div class="middle middle--alt">
if you could simplify your selectors to .middle and .middle--alt that's great, but it might not be the case as we don't know the rest of your css.
It's one of the possible 'proper' CSS way of solving your problem. However, if you can't edit the markup, see if you can use any cascading hooks (like in Praveen's answer) - otherwise you might have to use JS assuming that you can at least control what is loaded/executed on each page. Keep in mind that this kind of changes might incur in some FOUC (Flashing of unstyled content) due to the fact that your js will be likely applied after the dom is rendered and the css applied.