[Theme Horse Support Team]

Hi Weliajolijn,

Simply Go to Appearance -> Theme Options -> Design Options Tab -> Custom CSS paste the following CSS code and Click on save all changes button and your are all done.

#mc_embed_signup input[type="text"],
#mc_embed_signup input[type="email"],
#mc_embed_signup input[type="password"] {
    margin: 0 15px;
    width: 20%;
}
@media only screen and (max-width: 767px) {
    #mc_embed_signup input[type="text"],
    #mc_embed_signup input[type="email"],
    #mc_embed_signup input[type="password"] {
        margin: 0 0 30px;
        width: 100%;
    }
}

Hope this helps.
Thank you for using our theme.