You can edit the margins of your form, in the "Design" tab of your form:
Here is how to set them up:
Without a value, there will be no margin;
With a value (apx), this will define the margin for the four sides of the box;
With two values (apx ; bpx), the first will apply to the top and bottom sides and the second to the left and right sides;
With three values (apx bpx cpx), the first will apply to the top side, the second to the left and right sides and the third to the bottom side;
With four values (apx bpx cpx dpx), the first will apply at the top, the second at the right, the third at the bottom and the fourth at the left (which is clockwise, it is easier to remember 😄).
To go further
Do you want to apply your own CSS to the form? In this case, check the box: "Do not insert inline CSS"