I'm looking to swap the Flickr widget for this theme to Instagram. But I want the widget to look the same.
I use: https://codecanyon.net/item/instagram-widget-wordpress-plugin-for-instagram/11170758 for my Instagram widget but I don't know what custom css to use to make it match your theme.
External widgets has own custom CSS for their functionalities.
In this case you must contact the plugin author and tell him his plugin is not displaying the width properly (has a fixed width and it's not recommeded).
Maybe is somewhere in the admin you can set the width to 100% http://prntscr.com/nm4ezi
If you want to set the same design as Flickr widget, it will require several code customizations and that is not included within the support.
Here is another Instagram widget generator: https://lightwidget.com/ and you can paste the result code into an HTML widget.
Hope this may help you!