style.css

:root {
  --color: rgba(255,255,255,0.9);
  --tweet: white;
}

* {
  margin: 0;
  padding: 0;
  -webkit-box-sizing: border-box;
          box-sizing: border-box;
}

::-webkit-input-placeholder {
  color: var(--color) !important;
}

:-ms-input-placeholder {
  color: var(--color) !important;
}

::-ms-input-placeholder {
  color: var(--color) !important;
}

::placeholder {
  color: var(--color) !important;
}

body {
  padding: 50px;
  width: 100vw;
  height: 100vh;
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-align: center;
      -ms-flex-align: center;
          align-items: center;
  -webkit-box-pack: center;
      -ms-flex-pack: center;
          justify-content: center;
  background-color: #28a745;
  font-family: -apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Oxygen-Sans,Ubuntu,Cantarell,"Helvetica Neue",sans-serif;
}

.head {
  position: absolute;
  top: 30px;
  text-align: center;
  color: var(--tweet);
  font-size: 3rem;
  border-bottom: 2px solid var(--tweet);
}

#content {
  display: block;
}

#content input {
  color: var(--color) !important;
  margin: 1rem 0;
  width: 400px;
  border: none;
  border-bottom: 1px solid var(--color);
  font-size: 2.5rem;
  background-color: transparent;
}

#content input:focus {
  outline: none;
}

.hidden {
  visibility: hidden;
  opacity: 0.8;
}

.dark {
  --color: rgba(0,0,0,0.75);
  --tweet: rgba(0,0,0,0.95);
}

img {
  width: 24px;
}

#tweetBtn {
  position: absolute;
  bottom: 20px;
  left: 30px;
  text-decoration: none;
  color: var(--color);
  padding: 10px;
  border: 2px solid var(--color);
  border-radius: 5px;
  font-weight: 500;
}
#contribute {
  position: absolute;
  top: 0;
  right: 0;
}


@media only screen and (max-width: 560px) {
  #content input {
    margin: 0.75rem 0;
    width: 90%;
    font-size: 1.875rem;
  }
  #content img {
    width: 16px;
  }
  .head {
    font-size: 2rem;
  }
  #tweetBtn {
    left: 20px;
  }
  #contribute svg {
    width: 120px;
    height: 120px;
  }
}