I work in a company that has a lot of old applications and some of them are self-developed. The coder has retired and I have absolutely no connection to him whatsoever. He developed a web application to book vehicles and it is made up out of framesets. The frontend is an abomination and the users have requested a rework of it. Main reasons are lack of responsive design and as I said, it looks extremely bad.

I've tried to convert the old pages from the framesets into a CSS layout. After researching around the internet I chose an "include" way to do things. The problem is, the author has not thought about further developers at all and has made it significantly harder for them (me) to re-develop his application. Now, I am missing an approach to deal with these problems:

1. He passes variables through gets, that are directly opened in the frame using JS's window.open(). With include, they are opened sperately, but do not refer back to the site. For example, pressing "Send" on the login page will open a new page (blank / error), but you are logged in on the original site. This happens with every call of window.open(). With iframes, this happens as well. jQuery didn't fit my use case or I implemented it wrong - ($("#content").load("content.html");). Is there another possibility?
2. He uses functions to create divs, lables, buttons, input fields and all of these have a custom css built into the function. There are at least 200 function calls, so changing each and every call would be too much work. It is not possible for me to override the CSS, since it is nested so deep, through multiple files. Is the only solution * { unset:all;} or are there other possibilies?
3. Files that have been included in the source files of frames are included multiple times. Including them on the main page (e.g. index.php) and deleting them off the source files doesn't really work, since they need to have the functions seperately as well. To process the login for example, where the main page does not take effect.
4. Typical problems when modifying the structure of a program, like wrong file paths, undefined functions and variables etc.

I have done my research and tried some approaches to these issues (as explained above), but have failed and hope, someone can give me a heads up on how to approch this thing from a more experienced point of view. I am frankly frustrated and the fact that this is due next week does not add to my confidence at all.

I really appreciate any input. Thanks in advance!