Squeezed Content

On my WebSite all pages were built with default settings in such a way that I have a small padding left and right of the content (text, images etc.), which looked nice and was fine with me. Now, all of a sudden, the content is being squeezed quite a lot to the middle with a wide padding left and right of the content and left and right of the footer. The menue is still full width of the screen, which is ok. (refer to private content for an annotated screen shot). No clue what triggerd this unwanted change. On all pages where I use SmartSlider for showing images, the content, i.e. the image, is still full width of the screen – which is how it should be! – and the footer is squeezed in the same way as on the “normal pages” without SmartSlider (refer to private content for an annotated screen shot).
I have ckecked the following:
Sidebar settings: all sidebar options set to “no sidebar”
General Layout >> Layout >> Streched Layout: no change, if I change the layout type
General Layout >> Dimensions >> Maximum Container width: surprisingly, whatever I type into this field 500px, 5000px, 100%, 10% etc., it does not influence at all the width of what I see on my pages! I would have expected with 500px or 10% I would get an even more sqeezed appearance of my pages.
I found some coding in the forum how to influence the padding of the pages, i.e. force the padding to “0”, tried it by copying it the code into “Quick CSS”, but no change either.
Any idea where I have to look and what I have to do to get rid of the unwanted padding left and right and get back to how it was before?