Unable to make changes to menu

Does your theme use minified CSS files? If you edit the style.css and that file is not loaded (if theme loads style.min.css), you won’t see your change.
Or, your theme is loading that file, but the version parameter tells the browser to use the one in cache (it thinks it didn’t change). You would need to change the version number in the theme. This depends on how the theme loads the file, meaning the version parameter defaults to the WP version unless some version is referenced and that could be hard-coded or read from the top of the style.css file.

It’s best to make changes to your site without using the File Editors, though.
You can make a child theme if you have big changes. Or you can go to Customize > Additional CSS and put your CSS there. The CSS there will be output last, so it should override if you use the correct selectors.

Taking your suggestion, I’ve created a child theme. Can you help me out with the CSS code I should add?

No, because you didn’t link your site. You already know the CSS to change; you said you did it with Inspect. If you have theme CSS questions, it’s best to ask at the theme’s support forum, so it can benefit everyone. And the theme author knows the code the best (especially since menus are the trickiest CSS of the theme).