Can I achieve full width block if main page has margins?

Hi there, I’m getting to short strokes now. (BTW: Thanks for info on how to show hidden blocks. Helped me eliminate a lot of extra junk I’d put in without realizing it was there.)
Here is my next problem. I wanted to have a full width text box just below the header, but if I put in to the main content it ends up with same margins as main page.
So I put it as part of the header. But that means I can’t hide the box on certain pages (unless I create a separate header which I don’t want to do.)
How can I add the text box to the main page content area yet still have it full width in the content box?

Hi there, it seems I wasn’t clear enough in my question: I don’t want the content to be full width. I know how to do that. I WANT the content boxed.

My question is about achieving full width for my yellow text box. If I put the text box into the main content area (the ‘main wrapper’?), it inherits the margins to the right and left sides of the content box, which looks weird.

So in order to get the text box to go right to the edges of the content box, I, in effect, fooled things by integrating the yellow text box into the header, rather than the main content. But this means that, unless I create a separate header without the text box in it, that the text box will appear on every page, and there is some pages I don’t want it to appear on. If it’s in content, I can exclude it. But if its in header, I don’t think I can. (Or can I?)

So my question is: is there a way to keep the yellow text box not in the header, but in the main content box, while still enabling it to go right to the edges of the main content box, unlike the rest of the content?

Beautiful! Thank you. Always easy when you know how.