Shayla, I'm not sure if you can create a style-less theme for a form. I think what you may need to do is "trump" the other stylesheets by adding !important to any styles in the Marketo form. This is what we did.
You could try using a percentage in the css style for the form field width. (style="width: 50%;") It doesn't have to be a set pixel width.
Or perhaps you could also implement an @media set of styles into the Marketo form....which kicks into effect when a window is resized down (or viewed on mobile device). We did this to prevent fields from being too wide when viewed on mobile.
I hope this helps a little. I'd be glad to elaborate more if needed. I'm not a pro when it comes to CSS but I know just enough to be dangerous 🙂
To be clear, this isn't changing the markup, just disabling the CSS rules. In other words, if Marketo wraps a BUTTON in a SPAN, it's going to stay that way in the DOM. However you'll be able to style the markup any way you like.