Each page of your site references a couple of things. The corresponding html and css files for the particular php page your link connects to. So, join.php has in the first few lines some code which looks like this:
$_page['name_index'] = 3;
$_page['css_name'] = 'join.css';
this tells you that the html page it is referring to will be page_3.html in your /templates/tmpl_uni folder
the css file will be join.css in the /templates/tmpl_uni/css folder.
Now here's the rub. IF the php file is referencing an html page which DOES NOT EXIST in the templates folder, it is creating that html page on the fly from a default page. What you CAN do here is just take an html page which already has the basic appearance of the page you want to display, rename it to page_3.html or for the corresponding page it is referencing, and then tweak it to display how you would like it to. You can also do this with the css files. just change the code in the php file to join1.css and then copy the original css file and rename it join1.css and make the changes in it.
A lot of work, I know, but in the end you can customize your site just the way you want it to look by doing this.
Another tweak is you can change the headers for individual pages by copying and renaming the header file and adding or removing code in it, then reference the new header file. On my site, on the join page, I didn't want a member login or join now link showing up since they obviously were not a member and they were ALREADY on the join page, so I copied the original header file, removed the code for displaying this info, and then reneamed it header_join.html and from my new page_3.html, I referenced this new header. Now no join or member login shows up on just my join page.
Hope this helps!