discourse/app/assets/stylesheets/common/base/not-found.scss

// Page not found styles 

h1.page-not-found {
  font-size: $font-up-5;
  line-height: $line-height-medium;
}

.page-not-found {
  margin: 0 0 40px 0;
  &-search {
    margin-top: 20px;
  }
}

.page-not-found-topics {
  display: flex;
  @media screen and (max-width: 600px) {
    flex-wrap: wrap;
  }
}

.popular-topics, .recent-topics {
  box-sizing: border-box;
  width: 50%;
  max-width: 400px;
  margin-bottom: 40px;  
  padding-right: 20px;
  @media screen and (max-width: 600px) {
    width: 100%;
  }
  h2 {
    margin-bottom: 10px;    
  }
}

.not-found-topic {
  display: flex;
  flex-wrap: wrap;
  align-items: baseline;
  margin-bottom: 15px;
  @media screen and (max-width: 600px) {
    &:nth-of-type(n+6) {
      display: none;
    }
  }
  a:not(.badge-wrapper) {
    flex-basis: 100%;
  }
}