When you inspect elements on the affected pages, it should tell you which CSS file is enforcing the rule. If the file is somewhere inside /wp-content/plugins/ then a plugin is adding them and you can either disable that plugin or see whether they have properly enqueued the CSS. If inside the plugin the style is added like this:
wp_enqueue_style(...);
and you need the plugin’s functionality, just not the styles, you can create a child theme and inside its functions.php file add
add_action('wp_enqueue_scripts', 'wpse_325875_dequeue_plugin_css');
function wpse_325875_dequeue_plugin_css() {
wp_deregister_style(...);
}
(replacing the … with the same name the original plugin uses, so that it dequeues that particular file.)
Another possibility is that the dev before you may have modified Core files, which would be somewhere inside /wp-content/admin/ or /wp-content/includes/. Usually updating Core fixes this type of problem, but you can also manually download the .zip file from wordpress.org and overwrite your wp-admin and wp-includes folder. (Be careful not to overwrite wp-content as that contains all of your plugins as well as media uploads, like jpgs and pdfs.) That will ensure you’re using non-modified Core files.
As with any changes, make sure you have a complete backup of both files and database before making changes. 🙂