Hello,
how can I force a mobile header (with hamburger icon) from the width of 1200px?
Also, the header is now full width - contained for all sizes, but would like that the header is full width on resolutions lower than 1200px and full width - contained on resolutions larger than 1200px.
Your question seems to refer to CSS and media queries to adapt to different screen resolutions, not specifically a WordPress issue. You will probably get more response from the CSS category, so I am going to move the thread.
We would need to see the page in question (or at least a working demo) to determine what needs to be done as it will depend on your existing html, css and existing media queries (and possibly js).
It will require you being able to add your custom CSS and indeed manipulating the html if that is necessary.
Without a page to look at it will be hard to give further advice unfortunately.