To target a file in the images folders that is in the same directory as the css folder, it should rather be: background-image: url("../images/banners1.jpg");
The syntax:
A dot means one step up into a parent folder.
A slash means one step down into a named folder.
One dot and a slash before a file name means (up and then back) in the same folder.
Two dots and a slash before a folder name means a folder in the directory above.
Mind that the desired file name should finish the paths above.
or
2. Direct input:
href=“<?php bloginfo('template_url'); ?>/assets/css/styling.css”
and inside css
url(“/assets/images/banners1.jpg”); /assets will act as an absolute path in this case like <?php bloginfo('template_url'); ?>