Responsive Design for external PHP Page

Hi Guys,
i want to introduce myself, cause i am new at this place and need some help about Responsive Design.

My name is Denny, i live in Germany and helped my Wife with her Hompage as good as i can. I´m not new to building Homepages but in things like PHP, CSS and HTML in detail, i have not the routine only working on some issues time to time.

I administrated the Page of my wife and she have a Wordpreespage and use the great Theme “Vantage”.
She is a Midwife and use a external Software to organized herself. The software Company gave us some attachments to make a online registration ( include free Places and datetime) for her courses. So the custumor can see if a course available or not.

It worked verry well…but the external content is in an empty Page, only set is the “Theme” to the kursanmeldung.php.
The List of courses is set in an Shortcode on an other new Page. In the courslist (Kursliste)are the Information in one line each day. At the Registration page are a bunch of fields we need to know. The content are placed by default Design and did not floath if you visit the page by Phone or with a device with small resolution.

I want to fix that issue.
My question is where can i started to do so? Remember: The technical way are ok, but the Design or optical way are awefull.
My first idea is to create a DIV or CONTAINER via CSS. Another idea is to adjust the funtion.PHP that the external PHP page are known from the Theme and use the Responsive function too.

Maybe there are some advise to help me to keep this up.

regard and hopefully for some answers

Denny