How do I remove the margins on the sides of my website?

Question

I am trying to build a website for a class and for some reason the entire website doesn't touch the sides of the website. How do I fix this? The .main-content class is where it looks like the problem is, as all of the websites content apart from the header is located within it.

html {
  font-family: "Roboto", sans-serif;
  font-size: 16px;
}

.main-content {
  position: relative;
  top: 75px; /* To offset for fixed header. */
}

.center {
  text-align: center;
}

.button {
  border-radius: 5px;
  background-color: #4a4a4a;
  color: white;
}

.image-container {
  overflow: hidden;
}

.image-container img {
  display: block;
}

@media only screen and (max-width: 760px) {
  .main-content {
    top: 2.5625rem;
  }
}

I tried setting the left and right properties to 0, but they didn't change anything on the website.