main.css

html {
  font-family: Roboto, sans-serif;
  width: 100%;
  height: 100%;
}

body {
  margin: 0;
  padding-top: 4em;
  color: #000;
}

h1 {
  text-align: center;
  font-family: "Lexend Deca", Roboto, sans-serif;
  font-size: 30px;
  font-weight: normal;
  letter-spacing: -0.5px;
}

form, p {
  text-align: center;
}

p {
  font-size: 20;
}

input {
  margin: 10px;
  border: 1px solid #111;
  background-color: #eee;
  width: 150px;
  padding: 5px 5px 5px 5px;
  font-family: Roboto, sans-serif;
  font-size: 16px;
  border: 1px solid #ccc;
  height: 30px;
  border-radius: 5px;
}

select {
  margin: 10px;
  border: 1px solid #111;
  background: transparent;
  width: 150px;
  padding: 5px 35px 5px 5px;
  font-family: Roboto, sans-serif;
  font-size: 16px;
  border: 1px solid #ccc;
  height: 34px;
  -webkit-appearance: none;
  -moz-appearance: none;
  appearance: none;
  background: url(https://image.flaticon.com/icons/svg/118/118738.svg) 96% / 15% no-repeat #eee;
}

select::-ms-expand {
  display: none;
}

#isWeighted {
  margin-left: calc(50% - 90px);
  width: 180px;
}

@media (max-width: 1000px) {

  h1 {
    margin-top: -30px;
    margin-bottom: 5px;
  }

  input {
    color: #000;
    width: 100px;
  }

  #isWeighted {
    margin-left:calc(50% - 100px);
    width: 200px;
  }

  #calcGPA {
    width: 120px;
    margin-bottom: 5px;
  }

  div {
    font-size: 10px;
  }

  select {
    width: 120px;
    color: #000;
    margin: 10px 5px 10px 10px;
  }
}

@media (max-width: 500px) {
  div {
    font-size: 16px;
  }

  #calcGPA {
    width: 120px;
    margin-bottom: 5px;
  }

  select {
    width: 100px;
    color: #000;
    margin: 10px 5px 10px 10px;
  }
}