Category pages breaking in IE8, can anyone tell me why?

I’ve put together my own theme using the Toolbox theme and customising it, most things are working out so far, but I have a problem with IE8. I have a Mac and while writing the CSS I checked between Safari, Firefox and Chrome, but alas IE doesn’t like it. The category pages (https://terraavocado.com/category/uncategorized for example) lose their main content box when viewed in IE8, but display fine in every other browser I’ve tried. Can anyone tell me why? Note that I am very new to CSS, PHP and WordPress, and would really welcome any comments/criticism on anything I’ve done!

Thanks!