there are a couple of different approaches that you can use to apply your template to pages created by Data Assist
1) Use a Dreamweaver Template and apply it to the pages that Data Assist creates. On each page, go to Modify -> Templates -> Apply template to page.
2) Convert the template to a Framework Builder theme (open the template and go to WebAssist -> Framework builder Themes -> Create Theme), then open each page created by Data Assist and apply the theme (Go to WebAssist -> Framework Builder -> Themes -> Apply Theme)
3) Create a new page from your template (or theme if you go that that way), then add the pages created by Data Assist to it as a Plugin (Framework Builder -> Plugins -> Insert Plugin)
4) Similar to option 3, But use a Dynamic Plugin. this allows you to have one page in your site that pulls different content based on a variable like a Query string variable for example:
index.php?action=search <- pulls the results page
index.php?action=results <- pulls the search page
You may wish to use a combination of the methods above, dynamic plugins for the front end pages, but not for the admin pages for example.
On the Design Extender support page there is a tutorial for "Using themes in Data Bridge and Design Extender":
on the Data Bridge support page, the "Displaying Relational Tables with Plug-ins" gives some details on the basics of using plugins: