The simplest way is to do it with some CSS that you add to your landing page template, or better yet to a CSS file that you reference from your LP template.
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I'm a little confused here, mostly because #00FFFFFF doesn't signify transparency. If you're looking to do transparency, there are a few different ways to define it:
In fact, if you're talking about the eight-digit hex codes (that are so far only in Firefox 49+ and Safari), that'd be #FFFFFF00 (because you're still doing R, G, B and alpha transparency), but Marketo doesn't even support that yet (See Support RGBA output of mktoColor .)
All that aside, looking at your page, what is the outcome of what you're trying to do with the form?