- Use CSS clip-path property. Not supported by all browsers, read about support here . You can also overlay a repeating image, but the clip-path is preferred. You can make a banner immediately with triangles on top. Can be done using SVG. As they say, the taste and color. Still, it is preferable, again, the clip-path, which was created specifically for this.
- Using negative margins is generally a bad practice. The essence of the question is not entirely clear. What are the black blocks? Which div is "header"? You either lay out the code, or explain more precisely what is required, please.
As I understand it, you can’t place the basket and the balance on the top right of the fish. You can simply not merge your personal account with the header, then it will "go beyond" its limits. Of course, not everything is so simple, there will still be a lot to play with. But I don't even have a code to help you. Therefore, the details in the studio.