Border inside Grid Layout

One thing you can use the nth-child selector in a better way like below instead of targeting one by one.

.child:nth-child(-n+3) {
  border-bottom: var(--border);
}

.child:nth-child(3n+1) {
  border-right: var(--border);
}

.child:nth-child(3n) {
  border-left: var(--border);
}

.child:nth-child(n+7) {
  border-top: var(--border);
}

:root {
  --border: 2px dashed #393939;
  --symbol-color: #FF7F5B;
}

.grid {
  height: 100%;
  display: grid;
  grid-template-columns: repeat(3, calc(100%/3));
  grid-template-rows: repeat(3, calc(100%/3));
}

.child {
  display: flex;
  align-items: center;
  align-content: center;
  color: var(--symbol-color);
  font-size: 2.5rem;
}

.child:nth-child(-n+3) {
  border-bottom: var(--border);
}

.child:nth-child(3n+1) {
  border-right: var(--border);
}

.child:nth-child(3n) {
  border-left: var(--border);
}

.child:nth-child(n+7) {
  border-top: var(--border);
}

<div class="grid">
  <div class="child">1</div>
  <div class="child">2</div>
  <div class="child">3</div>
  <div class="child">4</div>
  <div class="child">5</div>
  <div class="child">6</div>
  <div class="child">7</div>
  <div class="child">8</div>
  <div class="child">9</div>
</div>

Since you want a stylized border (dashed, in this case), then your approach and the approach taken in the other answers appears to be useful.

However, if you decide to use a simple, solid line border, then the approach can be simplified. Just use the background color of the grid for border color, and the grid-gap property for border width.

.grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  grid-template-rows: repeat(3, 1fr);
  background-color: black;
  grid-gap: 1px;
  height: 100vh;
}

.child {
  background-color: white;
  display: flex;
  align-items: center;
  justify-content: center;
  color: #FF7F5B;
  font-size: 2.5rem;
}

body { margin: 0;}

<div class="grid">
  <div class="child"></div>
  <div class="child"></div>
  <div class="child">X</div>
  <div class="child"></div>
  <div class="child"></div>
  <div class="child"></div>
  <div class="child"></div>
  <div class="child">O</div>
  <div class="child"></div>
</div>

You may consider this workaround.

You may use grid-template-columns to do the trick.

• create a parent container that will hold your four images.

• set a background color (desire color of the border).

• set the padding to 0

• then do the trick arrange the images by grid-template-column: auto
  auto;

• then add gap to them grid-gap: 10px; (to show the background color of the container as grid).

please see code below for reference

.container {
  width: 200px;
  display: grid;
  grid-template-columns: auto auto;
  grid-gap: 10px;

  background-color: #000;
  padding: 0;
}
.container > div {
  background-color: #ccc;
  padding: 20px;
  text-align: center;
}

html

<div class="container">
      <div>Image here</div>
      <div>Image Here</div>
      <div>Image here</div>
      <div>Image here</div>
    </div>

to help you visualize i create a sample code

http://plnkr.co/edit/gIeumXLt0k3FPVCgGlDd?p=preview

Hope it helps

Cheers!