Most themes should have this built in. If you:
1) Log into WordPress
2) Using the left-hand menu navigate to “Posts”
3) Under the left-hand Posts click “Categories”
4) Hover a category name and click “View”
This will take you to the page where category posts are listed.
If you want to add files to your custom theme ( if it’s not a custom theme, make a child theme ) then you can copy or duplicate index.php
and rename it to category-{$your_category_slug_here}.php
.
For example, if I had category “Foo” the slug would be ‘foo’ and the template name would be: category-foo.php
. You can view more templates by following this URL.
To create CSS you could go to Appearance -> Editor -> style.css
but be very careful here. Another method is to use FTP and navigate to your custom theme to append your CSS to style.css