Plugin overwrites theme style

I had suddenly plenty of problems with my wordpress site until the theme’s author (muffingroup) wrote to many issues with an example:

“The problem is with mailchimp plugin. This plugin overwrite our theme styles and you should contact with plugins author to do not overwrite themes styles. Take a look at https://snag.gy/NhEhH.jpg to see where this comes from.”

What is your take on that after I invested so much time to integrate your plug-in …. all lost hours?

Please advice.

Sincerely,
STUDIO B?RIG

https://www.ads-software.com/plugins/mailchimp-for-wp/