How to change style of Custom Menu Widget?

I am using the Custom Menu Widget in the Bluebird theme. Even though it says, “The current theme does not natively support menus, …” the menu still shows up on the page.

The only problem is that it is inheriting some of the style from the main menu (which is also being used for the site). How can I get the Custom Menu Widget menu to use the same style as the Pages Widget menu?

I am assuming I would change one of the PHP files, but I have no idea how to do that or what code to put in there.

Here is the site: https://test.dosspta.org/

Thanks for any help!