How to change site margin on left-hand side only?

Site URL: http://calgarycannabis.squarespace.com

I added a vertical navigation, but the code I used isn't shifting the rest of the site over by 15px, which is the width of the nav. It looks off & is no longer centred as a result. Does anyone have a code which can shift the entire site over to the left by 15px?

Thank you!

calgarycannabis.squarespace.com
password: cannabis

Hi. Have you solved this yet?

Also, layout on tablet doesn't look good. 

/* tablet */
@media screen and (max-width:991px) and (min-width:768px) {
header#header {
    margin-left: 0 !important;
}
}