PHP in CSS file?

You can use wp_add_inline_style().

Let’s assume you enqueue your css like so

add_action( 'wp_enqueue_scripts', 'add_my_style' );

function add_my_style() {
   wp_enqueue_script( 'my_style', get_template_directory_uri() . '/mystyle.css' );
}

Then you can change the function like so:

function add_my_style() {

   wp_enqueue_script( 'my_style', get_template_directory_uri() . '/mystyle.css' );

   $style = "#mainnav li:after { content: url('{theme_url}/images/nav-divider.png'); }";
   $style .= "#another-example { content: url('{theme_url}/images/example.png'); }";

   $data = str_replace( '{theme_url}', get_template_directory_uri(), $style );

   wp_add_inline_style( 'my_style', $data )
}