Hi @du30,
This wordpress plugin is based on this jQuery plugin https://jackocnr.com/intl-tel-input.html
https://jackocnr.com/intl-tel-input.html has its own CSS and I simply import it from a CDN.
If you want to modify the styles, my suggestion is that you add another CSS file enqueued after the plugin’s file and modify the style. A easy way to do that is with jetpack. If you install this plugin, under “Appearance” in your Dashboard, you will have an additional CSS editor. You can write your styles there.
In this case, it is impossible to modify the CSS file because it is served from a CDN. However, in general, it is a bad idea to modify a plugin’s file. This is because, everytime a new version comes out and you update your site the old files are replaced by the new version and those wouldn’t have your modifications.
I hope it helps!
