Hi there,
first thing is to flush and disable the Autoptimize cache – you should leave this off when developing or customizing as it caches the CSS so your changes may not be visible. Also check in a Private Browser as this doesn’t cache and will show you the latest version.
You should not need to add the CSS – the documents Leo provided are how to change the container width and separating space.
If you increase the container width and the featured image has space around it then it is because the image is not wide enough to fill the space – you will need to upload a larger image.