.box {
  width: 900px;
  margin: 20px auto;
  display: grid; 
  grid-gap: 20px;
  grid-template-columns: 150px minmax(0, 1fr);
}

diarynav {
  border: #C5F5A4 4px solid; 
  border-style: double;
  grid-row: 1/2;
  grid-column: 1/2;
  height: max-content;
  background-image: url("https://64.media.tumblr.com/35af73a95ef47b4e83356006723d52a2/3f9ddb51d4bc096d-0d/s1280x1920/1c700e4bc8b6360e304a2ad49063b8edb6f4f48b.jpg");
}

diarybox {
  border: #C5F5A4 4px solid;
  border-style: double;
  grid-row: 1/2;
  grid-column: 2/2;
  height: max-content;
  background-image: url("https://dl.glitter-graphics.com/pub/3557/3557498d0um3u3vk0.jpg");
}