All solutions

So the website is finished on most of the desired screen size (responsive design). But there is one problem, it seems that when the website is being open at fullscreen browser, the content (including navbar and main content) will get stretched out to the side. On the other side, when its windowed, even on the max size of the screen it's not stretched out like before (this is what i want). A feedback or solution would be awesome about this issue, thank you in advance.

As this is my first project, I would love to get some feedback or even critics about my code, since i am new at programming, I would really appreciate all the feedback, especially at common mistakes on coding or even some minor mistakes that may become a bad habit. Thanks in advance.