A bit more information would be useful… for example there are hundreds of forms through Share, is there one form in particular that you want to change or are you trying to change the styling for forms overall (or something else?). When you say that you want to change the layout, what exactly are you trying to do?
You'll find most of the forms related code under "/Web Framework Commons/config/alfresco/site-webscripts/org/alfresco/components/form" in the source tree. The "/Web Framework Commons/source/java/org/alfresco/web/scripts/forms/FormUIGet.java" is the main class used by the forms webscript for getting the form data and most of the rendering is handled by macros defined in "/Web Framework Commons/config/alfresco/site-webscripts/org/alfresco/components/form/form.lib.ftl"
