offcanvas section's width itself is not hard to change with CSS. However, the horizontal offset of the main content when offcanvas is toggled seems to be set by javascript:
For clarifications's sake, on my mobile device the offcanvas section's width is 17rem, and the div#g-page-surround gets a inline-style of “ transform: translate3d(-272px, 0px, 0px)”. I can change the offcanvas section's width, but I couldn't find where to change div#g-page-surround's translate value accordingly.