You can…but it means using the child theme to make core theme file changes. To begin with, you would copy over the index.php into the child theme and look for the class span8 and change this to span9.
Next do the same for the sidebar-blog-left.php and look for this class span4 and change it to span3. Repeat this for the the sidebar-page-left.php as well if you want the pages to have this modification and not just the blog pages.
There are other files with these classes, but if the ones mentioned here are the main ones for the default blog layout (left sidebar) and the page one.
The child theme for this theme is found in the theme folder “child-theme” that you can install. Please note when you activate this that you will need to redo your theme options and reassign your widgets.