Remove grey border, pictures wider than text, logo to the center

Hello, I’m using Wallpress theme and i want to remove thin grey lines(border) around posts and to make pictures inside post wider than text column (For example text column is one third of the screen width but pictire full screen width) But I cannot find the code for this, only for border around images. Now all pictures are sized into column with this grey border. And I would like to place logo in the center of the header menu, now it is aligned to the left.
Thank you