[Theme: Third Style] Change the width of the "Third style" theme

edit the style.css of the theme;

please post a link to your site using that theme.

you can try and use a tool such as Firebug https://getfirebug.com/ to identify which styles to edit.

there seems to be a section:
#access .menu-header, div.menu, #colophon, #branding, #main, #wrapper {...}
where the width is set to 940px .
but there might be more styles to edit.

if you are not successful using Firebug, you can try and go through style.css line-by-line, and adapt all widths in the region of 940px.