Category Page Layout not same as Posts Page Layout

This is on a site that I’ve been asked to fix.
Why is the posts category page displayed with a different layout to the blog page, and the individual posts page?
When it’s viewed through Inspect Element, on the Blog and Posts page there is an Inline Element with .css setting the width and margin-left.
This element is not there on the category page.
There is a separate category.php file with the same site-content code as index.php.
Where do I find where this is set, and how to fix it so that all post related pages have the same width (other normal pages should be full width)?

Inline element that’s missing off the category page
.site-content {
width: 60% !important;
margin-left: 50px !important;

http://www.embracingyourfifties.com/blog/ (correct layout)
http://www.embracingyourfifties.com/category/anxiety/ (wrong layout)
http://www.embracingyourfifties.com/confidence/3-secrets-to-deal-with-self-confidence-issues-that-affect-every-aspect-of-your-life-part-8/ (correct layout)