../assets/[...] relative link to your styles in
embeds/header-inner. That'd work from
http://localhost/index.php/whatever/you-want/ or from
http://localhost/blog, but not from anything that looks to the browser like a deeper folder structure (
http://localhost/blog/categories/[something]) those wouldn't work.
Try something from the site root if you can, like
<link rel="stylesheet" href="/assets/css/file.css">.