style.css

:root {
  --background-dark: #7c3aed;
  --black: #000;
}

html {
  background-color: var(--background-dark);
  color: var(--black);
  height: 100%;
}

* {
  font-family: "Lato", sans-serif;
}

.header {
  text-align: center;
  margin-top: 100px;
}

.container {
  width: 300px;
  margin: auto auto;
}

label,
input {
  display: block;
  margin-top: 0.5rem;
  width: 100%;
}

label {
  font-weight: 500;
  font-size: 1.3rem;
}

input {
  border-radius: 5px;
  padding: 0.5rem;
  border: transparent;
  font-size: large;
  margin: 1rem 0;
}

#calculate-number {
  margin: 0.5rem auto;
  display: block;
}

#calculate-number {
  background-color: var(--background-dark);
  border: 2px solid var(--black);
  color: var(--black);
  padding: 0.4rem 1rem;
  border-radius: 0.5rem;
  font-size: large;
  cursor: pointer;
  outline: var(--black);
  margin: auto;
  display: block;
}

#output-container {
  display: block;
  margin: auto;
  margin-top: 1rem;
  width: 100%;
  text-align: center;
  padding: 0.5rem;
  font-size: x-large;
  border-color: var(--black);
  border-width: medium;
  border-radius: 5px;
}

/* footer */

footer {
  text-align: center;
  position: absolute;
  bottom: 0;
  left: 0;
  right: 0;
}

footer p {
  font-size: large;
  text-align: center;
}

.footer-list {
  margin: 1rem;
}

a,
img,
li {
  display: inline;
}

img {
  width: 40px;
  padding: 0.2rem;
}