demo.css

@import url('https://fonts.googleapis.com/css?family=Montserrat:100i,300,400,500,800&display=swap');
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

h1, h2, h3, h4, h5, h6 {
  font-family: 'Montserrat', sans-serif;
}
li {
  list-style: none;
}

input:hover {
  border: 1px solid blue;
}
svg {
  width: 1rem;
}

body {
  display: flex;
  flex-direction: column;
  justify-content: space-between;
}

img {
  margin-right: 1rem;
  margin-bottom: 1rem;
  float: left;
}

header {
  border: 2px solid purple;
  display: flex;
  justify-content: space-between;
}

nav ul {
  display: flex;
  flex-direction: row;
  justify-content: space-between;
}
nav {
  min-width: 30%;
}

.single-dog-card {
  padding: 1rem;
  flex-basis: 20em;
  border: 2px solid peru;
  border-radius: 1rem;
  margin-top: 1rem;
}

.dog-list {
  display: flex;
  flex-direction: row;
  flex-wrap: wrap;
  justify-content: space-around;
}

main {
  border: 2px solid turquoise;
  flex-grow: 2;
}

header, main, footer {
  padding: 1rem;
}
footer {
  border: 2px solid peru;
}