simple-keyboard/src/demo/css/ButtonThemeDemo.css

input {
  width: 100%;
  height: 100px;
  padding: 20px;
  font-size: 20px;
  border: none;
  box-sizing: border-box;
}

.simple-keyboard {
  max-width: 850px;
}

.simple-keyboard.my-theme .hg-button.my-button {
  background: blue;
  color: white;
}

.simple-keyboard.my-theme .hg-button[aria-label] {
  background: black;
  color: white;
}

.simple-keyboard.my-theme .hg-button.my-button-outline {
  border: 2px solid red;
}