Hi 1 – I have your theme but i can‘t place the logo in the middle of the header. How can I do ? You need to alter the header.php and use css custom rules to center that . More exactly css class .site-branding,img.custom-logo and .main-navigation Unfortunately this is not something i can do fast so i cannot offer you any code.
2- I want more than 3 items on my « wp Stream items slider. How can I do that please ? The slider – i assume you are talking about the front end slider (the one on home page http://prntscr.com/s8fsg2 ). If yes you need to go into theme admin and add the new product in the theme slider section http://prntscr.com/s8frdi . Make sure the items you add have an image and/or video attached.