Hi Dani,
I do not have any link. This is easy reproducible.
1. In Admin Dashboard, go to MailPoet → Forms and click on New Form
2. Do not select any template, just click on Create blank form above the templates
3. Save the form and add it to any page in your WordPress
4. Try to change padding, margin or border-color of the submit button via theme settings. You will fail.
The reason is: those three settings (padding, margin and border-color) are added by MailPoet to the form via style attribute.
If I create a blank form, where I choose to use the theme style, I do not expect any style settings added by MailPoet.
Thanks!