I put the subscription form widget in my footer. My footer is coloured to match my corporate design.
But now you can’t really see the text fields or drop down options.
Is there a way to change the style of these fields? maybe to colour them white or put a thicker frame around the text field?
You should add custom CSS. You can add them in the main Newsletter config panel, you can ask the designer to create a set of css for you. Or, if that is the standard from rendering, ask the designer to (gelerally) deal with form rendering in the footer.