In order to maintain the highest levels of security, we do not recommend embedding your Donation page into a website via an iframe. First, if the website is not accessible via HTTPS (SSL) then, when potential donors come to your donation page, they may be concerned about providing their credit card information on a page that does not have padlock icon appearing in their web browser. Second, from a technical perspective, doing so can allow for a man-in-the-middle attack where an attacker could potentially collect your donor's credit card information.
If you'd like to have your Soapbox Engage template look like your website design to provide a seamless transition from it to your Soapbox Donations page, just open a support ticket requesting a quote for a custom Soapbox Engage template, and our professional services team can help you.
Happy online engagement!