See how the parent categories now stick out and better display the hierarchies? Below you will find the steps you need to take to make this change. Some of you can skip step #1 if you’re already using a custom.css file on your site.
Navigate to your /styles/ directory and rename custom-sample.css to custom.css
Open custom.css in your favorite code editor
Enter the style code below and save
Upload the file to your web site
All this does is tell your site to bold any level-0 (parent categories) within your drop-down lists.
That’s it. You might need to flush your browser’s cache (Shift + reload in Windows) and/or clear any WordPress caching plugin you are using before the change will show up.