styles.css

* {
  margin: 0; padding: 0;
  background-color: var(--black);
  text-align: center;
  font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
  color: var(--white);
}

input::-webkit-outer-spin-button, input::-webkit-inner-spin-button, input[type=number] {
  -webkit-appearance: none;
  -moz-appearance: textfield;
}

body { min-width: 350px; }

header {
  height: 40px;
  min-width: 260px;
  margin: 10px 0;
  border-bottom: 1px solid var(--black_alt);
  padding-bottom: 10px;
}

main {
  display: flex;
  flex-direction: column;
  align-items: center;
  min-height: calc(92vh - 40px);
}

footer {
  display: flex;
  justify-content: space-around;
  border-top: 1px solid var(--black_alt);
  padding: 10px;
}

#options, #light-frequencies {
  display: flex;
  flex-direction: column;
  align-items: center;
}

#options { margin-bottom: 10px; }

#buttons {
  display: flex;
  justify-content: space-around;
  width: 100%;
  max-width: 300px;
  margin-bottom: 10px;
}

h1, h2 { color: var(--white_alt); }

#img-uploaded > img {
  max-height: 200px;
  margin-top: 10px;
}

label { margin: 5px 0; }

input, select {
  width: 100%;
  max-width: 200px;
  padding: 1px 5px;
  border-radius: 5px;
  text-align: left;
  border: 1px solid var(--black_alt);
}

input[type=checkbox] { width: auto; }

.frequency { width: 40px; }

button {
  width: 100%;
  padding: 5px;
  margin: 5px;
  border-radius: 5px;
  border: 2px solid var(--black_alt);
  color: var(--black_alt);
  font-weight: bold;
}
button:not(:disabled) { cursor: pointer; }

#listen-button:not(:disabled) {
  border-color: var(--green);
  color: var(--green);
}
#listen-button:hover:not(:disabled) {
  background-color: var(--green);
  color: var(--black);
}

#stop-button:not(:disabled) {
  border-color: var(--red_alt);
  color: var(--red_alt);
}
#stop-button:hover:not(:disabled) {
  background-color: var(--red_alt);
  color: var(--black);
}