Unfortunately, I can't find the "the extra CSS field available in the subscription panel". If I go under List creation>subscription>General there is no CSS field. If I go under the "subscription" tab, there is just a shortcode but no CSS field.
I found a way to do it. Went to "Listing creation" then "Fields, buttons of the subscription form", then "Form code". I copied the "Widget form code" at the bottom then went to "Appearance">"Widgets" of the website. I then added a "Custom HTML" widget, pasted the code there and, next to type="submit" at the end of the code, just added a style="background-color:#0092ff" and voila!