Hi, mabena!
The plugin tries to use your website’s themes styling (CSS) as much as possible, so it does not look out of place on your website.
If you are comfortable coding you can change or add CSS to your website that will give you the desired effect.
If you are interested in the CSS that Easy Sign Up adds to your website have a look at these links:
https://www.beforesite.com/docs/easy-sign-up-default-css/
https://www.beforesite.com/2015/02/03/customise-easy-sign-form/
If you’re not confident with writing CSS feel free to post a link to the web page with the form and I’ll post a snippet of code here.
Also did you know that the JetPack plugin adds a CSS editor tool to your admin area, the great thing about this is you don’t need to mess with your theme’s CSS. Pressing that update Theme button will not cause you to lose your modifications!