You will have to edit all ASP files in web enrollment site pages and edit HTML layout. They are stored in %windir%\system32\CertSrv\en-US
. Unfortunately, web enrollment doesn't use master-page concept (which is possible in ASP), ADCS web enrollment does not implement it, so you will have to edit ALL asp files.
However I would re-evaluate the necessity of legacy web enrollment. It is greatly outdated and most of features are available only in Internet Explorer because web site uses ActiveX. Keep in mind that Internet Explorer is discontinued by Microsoft.