style.css

html, body {
  height: 100%;
  margin: 0;
}

body {
  display: flex;
  flex-direction: column;
  justify-content: flex-start;
  align-items: center;
}

.options {
  background: #ddf;
  display: flex;
  justify-content: center;
  flex-wrap: wrap;
  width: 100%;
}

.options h1 {
  font-size: 1.2rem;
  margin: 0;
  display: block;
  width: 100%;
  text-align: center;
  text-decoration: underline;
}

.options label {
  padding: 1em;
}

main {
  background: #ffd;
  width: 100%;
  height: 100%;
  display: flex;
  justify-content: center;
  align-items: center;
}

.container {
  width: 70vmin;
  height: 70vmin;
}

.target {
  display: flex;
  justify-content: center;
  align-items: center;
  border-radius: 100%;
  position: relative;
}

.target.flash::after {
  content: '';
  width: 100%;
  height: 100%;
  border-radius: 100%;
  position: absolute;
  top: 0;
  left: 0;
  background: none;
  animation-name: flash;
  animation-duration: 0.1s;
  animation-iteration-count: 3;
}

@keyframes flash {
  from { background: none; }
  to   { background: yellow; }
}

.outer {
  background: #800;
  width: 100%;
  height: 100%;
  z-index: 10;
}

.middle {
  background: #080;
  width: 70%;
  height: 70%;
  z-index: 20;
}

.inner {
  background: #008;
  width: 40%;
  height: 40%;
  z-index: 30;
}