Mobile website

Obviously it is that <table> with 700px fixed width inline style.

You mean the text on the right hand side?

Yes, and content on the left too, they are both in a <table> with a fixed 700px width.

There are other things too. I tried removing (via Firebug) all other stylesheets and left in only main (parent) active, the site looked wrong. So the main theme stylesheet had been edited making it difficult to debug. You should have it untouched and keep all the changes in CSS in one place, make a desicion whether to use child stylesheet or a Custom CSS plugin.

The logo must have responsive image CSS (100% width, auto height) applied too.

Thank you for your reply.
I would like to use the child stylesheet, but I don’t know how to copy the code to the childtheme.

It’s not literally copying to child theme.

To not interfere with the live site, install a local dev somewhere and clone the site into it. Then extract all the changes into child theme stylesheet. You do exactly the same like you did before but this time instead of directly modifying the parent stylesheet, you do it via child theme by override/overwrite only changes in CSS.

If you know your way around CSS and there are not many changes in the parent stylesheet (like you remember what you did), skip the local dev and work in Firebug and update the live site directly when ready.

I reinstalled the original stylesheet. But I am not able to make the changes in the website to make the site look good on a mobile