Hi there, I had a look at the way the site acts responsively at smaller screen sizes to get started, because I noticed that when the browser window gets smaller (starting at 700px wide), the layout changes to the one you want: one column with a sidebar. This is the relevant piece of CSS:
/* Large posts reduced to one column plus sidebar */
@media screen and ( min-width: 700px ) and ( max-width: 1180px ) {
.two-column #main {
width: 488px;
}
.two-column.left-sidebar #main,
.two-column.right-sidebar #main {
width: 688px;
}
.two-column #primary {
width: 488px;
}
}
You can you remove the max-width value from that bit of code to get the same layout on larger screens as well. Try adding this to your custom CSS:
@media screen and ( min-width: 700px ) {
.two-column #main {
width: 488px;
}
.two-column.left-sidebar #main,
.two-column.right-sidebar #main {
width: 688px;
}
.two-column #primary {
width: 488px;
}
}