Hi MutebiRoy,
Thanks for hopping in the forum. To provide some background, the plugin is separate from the form that is designed on the MailChimp site. So the styling would need to be added the css of your website. On the plugin’s settings page, there’s a great cheatsheet that has the css classes and id’s you would need to style. To reach that page, in the WordPress admin area look under Settings > MailChimp setup. If you don’t have html or css coding knowledge, it might be best to consult your web developer to help with this.
As an alternative, you can embed the entire MailChimp signup form into your website, this would include all the style that you designed. This guide from the MailChimp explains doing that. https://kb.mailchimp.com/article/how-can-i-add-my-signup-form-on-my-website/#fullform
-mcd