Some .CGI or .pl files, may contain a Form tag like,
<form action="script_sile_name.cgi" method ="GET"> or <form action="script_sile_name.cgi" method ="POST">
This means that you can directly open this file in your browser and you can change its appearance with your favorite HTML editor. Some scripts usually come with html templates. You can open the html file in an WYSIWYG editor like Dreamweaver or Frontpage. But if there is no template provided by the scriptwriter then you need to open and edit the .cgi or .pl files after installing and testing them successfully, you may find that they are generating some html files dynamically. Hence you wouldn't have much control over how the script appears on your page. If you attempt to modify the script it will be at your own risk.
If you are good in programming, and are courageous enough to change the script, you can try to locate and change the necessary html properties like color or font defining the output html document. But it's not advisible to modify the script if you are not sure of what you are doing. When you feel that the script is not flexible enough to meet your needs, you can change to another script. Sometimes we don't bother about the script's flexibility if its functions are great. In that case,you may need to give up on the look and feel.