Weebly amend CSS

I have a website made on Weebly and I need to make a change to the CSS to remove the padding/margins of an embedded custom html code in one the pages.
- This is the form outside of Weebly (attached screenshot)
- and again the form inside Weebly (attached screenshot ) you can see the big spaces between the lines which I think are caused by the padding/margins issue.

It is a very simple job, but you need to know very well exactly what you are doing.