So, I have tried everything I can think of to get rid of the white margins around everything on my page.
I have put: margin: 0;  on everything I can think of and it still does not get rid of it. Any help would be great!
I apologize for the giant wall of code.
function myFunction() {
  var x = document.getElementById("myTopnav");
  if (x.className === "topnav") {
    x.className += " responsive";
  } else {
    x.className = "topnav";
  }
}
/* global */
* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}
html {
  margin: 0;
  padding: 0;
  width: 100%;
  height: 100%;
  font-family: sans-serif;
}
body {
  margin: 0;
  padding: 0;
  width: 100%;
  height: 100%;
  font-family: "Tahoma", sans-serif;
  font-size: 16px;
  color: #454545;
  background-color: #fff;
}
div {
  margin: 0;
}
/* end global */
/* custom */
.container {
  padding-right: 15px;
  padding-left: 15px;
  margin-right: auto;
  margin-left: auto;
}
.footer {
  padding: 5px 10px;
  background-color: #428cd9;
}
/* end custom */
/* custom responsive */
.row::after {
  content: "";
  clear: both;
  display: block;
}
[class*="col-"] {
  float: left;
  padding: 15px;
}
.col-1 {
  width: 8.33%;
}
.col-2 {
  width: 16.66%;
}
.col-3 {
  width: 25%;
}
.col-4 {
  width: 33.33%;
}
.col-5 {
  width: 41.66%;
}
.col-6 {
  width: 50%;
}
.col-7 {
  width: 58.33%;
}
.col-8 {
  width: 66.66%;
}
.col-9 {
  width: 75%;
}
.col-10 {
  width: 83.33%;
}
.col-11 {
  width: 91.66%;
}
.col-12 {
  width: 100%;
}
/*end custom responsive */
/* navbar */
ul.topnav {
  list-style-type: none;
  margin: 0;
  padding: 0;
  overflow: hidden;
  background-color: #428cd9;
}
ul.topnav li {
  float: left;
}
ul.topnav li a {
  height: 55px;
  display: inline-block;
  color: #454545;
  padding: 0px 16px;
  line-height: 3.0;
  text-decoration: none;
  text-align: center;
  font-size: 17px;
  transition: 0.3s;
}
ul.topnav li a:hover {
  background-color: #2162a6;
  color: #757575;
}
ul.topnav li.icon {
  display: none;
}
/* end navbar */
/* responsive navbar */
@media screen and (max-width:680px) {
  ul.topnav li:not(: first-child) {
    display: none;
  }
  ul.topnav li.icon {
    float: right;
    display: inline-block;
  }
}
@media screen and (max-width:680px) {
  ul.topnav.responsive {
    position: relative;
  }
  ul.topnav.responsive li.icon {
    position: absolute;
    right: 0;
    top: 0;
  }
  ul.topnav.responsive li {
    float: none;
    display: inline;
  }
  ul.topnav.responsive li a {
    display: block;
    text-align: left;
  }
}
/* end responsive navbar */
@media screen and (max-width:680px) {
  .nomobile {
    display: none;
  }
  .yesmobile {
    width: 100%;
  }
}
.header-img {
  min-height: 300px;
  background-image: url(http://thirdshiftdesigns.net/images/cabheader2.png);
  background-repeat: no-repeat;
  background-size: auto 100%;
  background-position: center top;
  /*padding: 40px; (If don't want to set min-height or some image content is there) */
}
.end-header {
  width: 100%;
  height: 15px;
  background-color: #428cd9;
}
<body>
  <div class="col-12">
    <ul class="topnav" id="myTopnav">
      <li><a href="#">Logo</a></li>
      <li><a href="#">Home</a></li>
      <li><a href="#">About</a></li>
      <li><a href="#">Contact</a></li>
      <li class="icon">
        <a href="javascript:void(0);" style="font-size:15px;" onclick="myFunction()">☰</a></li>
    </ul>
    <div class="row">
      <div class="col-12">
        <div class="header-img">
        </div>
      </div>
    </div>
    <div class="row">
      <div class="col-12">
        <div class="end-header">
        </div>
      </div>
    </div>
  </div>
  <!-- end header -->
  <!-- footer -->
  <div class="col-12">
  </div>
</body>
                        
If you inspect the element, you can find that the unwanted space is because of the following style which applies
paddingto all elements with theclassvalue containingcol-.Override the style and you can get rid of the unwanted space. Note that this will set the
paddingto 0 for all the classes whose value containscol-.Or you can only override the
paddingof.col-12which will apply padding of 0 to.col-12while the other classes containingcol-will still get a padding of 15px.