First, actually consider your security. Is your form safe from reasonable levels of attack? If there's a chance data could get out, stop and fix that first. Are you SSL certified? Are your POSTing your data? Are you taking all reasonable steps to ensure safety of the data?
Anyway, onto the problem at hand. If you've got a design scheme, as a general rule you should stick to it. If you haven't drastically changed anything else on the page, I recommend keeping with your scheme: your users are used to it on your site and actually, changing it may get them more suspicious than keeping it the same.
There is a small risk that people may think that it's 'insecure'. However, many people are 'trained' (for want of a bettter word) to assess security for themselves using traditional indicators such as the padlock symbol and https: protocol. From a user's standpoint, I wouldn't be concerned about the security of the form as much as the reputation and intentions of the company behind it. What you may want to do is add a little caption at the top saying 'All data you give us is confidential and will only be used for the purposes we describe. It is not shared with third parties.' As far as allaying suspicion is concerned, that's about the best a UI designer can do.
To elimimate the possibility of people thinking it's insecure, make sure it isn't. That way all the traditional indicators people look for are there.
Hope this helps.