Unfortunately, it's not quite as simple as just changing the width on that container. The CSS does not contain media queries for breakpoints beyond the current maximum column width. Consequently, the layout may break at wider column widths, requiring a significant amount of work beyond simply making the column wider. It would also result in wider content areas with longer lines of text, which are generally more difficult to read. I'm not ruling out further theme options in the future, and certainly, if there is support for such a change from the wider CADTutor community, I'd be happy to consider this for a future update.