How to customize this theme? [closed]

Move the Navigation to a new Line:
Add clear:left; to the .main-navigation class in your styles.css

With Widgets I assume you mean the 3 sections with the blue graphic.. those have a horizontal padding of 25px and are evenly distributed by their width of 33.33333% -> look at the class .sow-features-list .sow-features-feature

Line 21 of your styles.css

.sow-features-list .sow-features-feature {
  text-align: center;
  float: left;
  -ms-box-sizing: border-box;
  -moz-box-sizing: border-box;
  -webkit-box-sizing: border-box;
  box-sizing: border-box;
  padding: 0 25px; //change this line
  margin-bottom: 40px;
}