Questions about changes in CSS!

Dear, I have some questions. Please can you help me?

1 - In my site, I put a full width slider. But, the area has a margin (or padding) between the header and the next section. I didn't found a way to change this. I put margin and padding ZERO in every region CSS but they didn't disappear. I put a GREEN background for anyone that want help me can see...

2- Is it possible change the size of grid??? Is to big for me...

3- In every image that I put, they put a margin (or padding) in this image. Is there a way to change this too?

I really need help with this issue!!!!