Do you mean like the buttons on this page: Află cum funcţionează Serviciul de Email-Marketing aproape-GRATIS » Ionuţ Ojică ?
Those Forms are both from Mautic. The styling of the button is done within a custom Mautic template. All I have to do in Mautic is to choose the right template.
Perhaps you find relevant this post: How to style mautics generated forms? - #5 by dirk_s
I created the template, so I will stick with the template for the moment.