All these snippets go in to the Dashboard -> Appearance -> Customize -> Additional CSS.

Greater gap between Menu Items

.et-l--header .et-menu-nav>ul>li {
     padding: 0 30px!important;
 }

Adjust the 30px to what you want.

Make Post Section Transparent

.single-post #main-content {
 background-color: transparent;
 }

Blog Preview on Large Screens

Had an issue using “Blog Settings”, where I naturally had three across on smaller screens, 1600px wide, but on HD, 1920px, it became two wide and that misplaced on the screen. To fix this Sergey from Divi got me a solution. First go to the Divi -> Theme Builder and in your Blog Template in the Blog Settings -> Advanced -> CSS ID & Classes -> CSS Class add

cu-blog

Then in Divi -> Theme Options -> General -> Custom CSS add

@media only screen and (min-width:981px) {
    .cu-blog .column.size-1of3 {
        width: 29%!important;
    }
}