Form 2.0 comes with several themes but none of them matches or work well with our brand. When I use the embedding code to load a form to a page, it loads the style from Marketo as well. I had to write a lot of CSS code to overwirte the styles applied to the form.
Marketo's approach makes form creation easier for non-designer and non-technical users, but it a world where design and branding are huge drivers of conversion, allowing developer to add custom class to form elements so they become compatable with CSS framework like Bootstrap and foundtation can save a lot of production time across industry, no to mention pushing Marketo's reach to companies and take brand and creative as piority.
If for any reason allowing custom class is a technical challenge and security risk, at least provide a unstyle version so developers don't have to spend so much time on overwriting existing CSS styles.