Hi @tmora!
Changing the color or the size are already changes that some providers do not allow at all, as they have the exact guidelines for the color, font, font size, background color, paddings to be used. In case you really want to change the design and risk your app getting disabled, then for CSS you do not need to search for a folder, you just need to check the buttons on a page where they are published (by default on the /wp-login.php page we will display the buttons for the enabled providers), then write your custom CSS based on the selector of the button, and use the !important rule if needed. Also, please note that we are unable to provide support for custom coding and problems that are the result of such custom coding.
Note: On the Buttons tab you can change some parts of the HTML of the given provider’s button:
https://nextendweb.com/nextend-social-login-docs/buttons/
Or if you want, you could make custom HTML buttons as well, all you need to do is add our link around your HTML, as you can see on the Usage tab for each of the providers:
https://nextendweb.com/nextend-social-login-docs/usage/
In this case it is important to mention that these links will be hard-coded, so they will not track our endpoints, so if our endpoint URL changes (for example the /wp-login.php slug changes, or you configure the OAuth redirect uri proxy page), you will need to update the links manually as well.