Hi Val,
You should be able to just put <style> tags on the page and have it apply to the various IDs or classes on the page, just like one could with the original form editor.
For example, when I dragged an HTML block onto a page (this should work the same with a non-marketo page as it does with a marketo page), and added the following:
#FirstName {
font-size: 25px;
}
It gave me this: