Can I change the font, font-size or colour of the form fields?

No, I’m afraid you can’t. I am quite limited in what fonts I can use for the forms since I can only select fonts that will be on everyone’s computer.

That’s why I have opted for Helvetica because I think out of the available options it looks best. The font size will be around 12px unless you have a lot of text in a field, in which case it will make the font smaller to fit all the text into the field.