This is something someone has to be able to see. You have to right click the form in Chrome, click "Inspect" and a window will pop out on the right. There you will see the CSS codes for the form. You have to copy those codes and paste them into Appearance/Customise/Additional CSS ... then edit and adjust them.
The editing and adjustments will depend on what form you are using and how you want it to look.
It's impossible to tell you how to do it exactly without being able too see it and without knowing how you want it to look. It's like trying to make a tailor made shirt someone you can't see and not knowing what style they like.
You need to learn how to do it yourself or get someone to do it for you.
In the popout console when you click "Inspect", look for codes like this...
mc_embed_signup
mc-embedded-subscribe-form
Then you need to change padding, borders, height, text, colours etc, etc.