Override Theme CSS with CSS from a plugin

You can try to dequeue the plugin stylesheet, then re-enqueue the stylesheet with your theme stylesheet added as a dependency which will ensure that the plugin stylesheet is loaded after your theme stylesheet.

function custom_style_order() {

    if ( defined('JOB_MANAGER_PLUGIN_URL') ) {

        //dequeue the original plugin stylesheet by its handle
        wp_dequeue_style( 'wp-job-manager-frontend' ); 

        //re-enqueue the stylesheet but with an added dependency (your theme stylesheet)
        wp_enqueue_style( 
            'wp-job-manager-frontend', 
            JOB_MANAGER_PLUGIN_URL . '/assets/css/frontend.css', 
            array('your-theme-stylesheet-handle') 
        );

    }

}

add_action('wp_enqueue_scripts', 'custom_style_order', 100); 

Note: you will need to find the “handle” of your theme stylesheet, as an example if we are dealing with a default theme like Twenty Twelve, then the handle for that themes main stylesheet is twentytwelve-style found in its functions.php file on line 150.