style.css

:root {
  --color-black: #1b1b1b;
  --color-white: #fff;
  --color-green: #16A085;
  --color-red: #C82647;
  --color-orange: #EB9532;
  --color-purple: #7E3661;
}
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
body {
  width: 100vw;
  height: 100vh;
  background-color: var(--color-black);
  font-family: Verdana, Geneva, Tahoma, sans-serif;
  display: flex;
  align-items: center;
  justify-content: center;
}
button {
  background-color: transparent;
  border: 2px solid;
  border-radius: 10px;
  cursor: pointer;
  margin: 0 20px;
  padding: 1.2rem 2.8rem;
  font-weight: bold;
  font-size: 1.4rem;
  color: var(--color-white);
}
button:focus {
  outline: 0;
}
.btn_1 {
  border-color: var(--color-green);
  transition: box-shadow 0.2s ease-in-out;
}
.btn_1:hover {
  box-shadow: 0 0 40px 40px var(--color-green) inset;
}
.btn_2 {
  border-color: var(--color-red);
  box-shadow: 0 0 40px 40px var(--color-red) inset;
  transition: box-shadow 0.2s ease-in-out;
}
.btn_2:hover {
  box-shadow: 0 0 10px 0 var(--color-red) inset, 0 0 10px 4px var(--color-red);
}
.btn_3 {
  border-color: var(--color-orange);
  background-image: linear-gradient(45deg, var(--color-orange) 50%, transparent 50%);
  background-position: 100%;
  background-size: 400%;
  transition: background 0.2s ease-in-out;
}
.btn_3:hover {
  background-position: 0;
}
.btn_4 {
  border-color: var(--color-purple);
  position: relative;
  overflow: hidden;
  z-index: 1;
}
.btn_4::after {
  content: "";
  position: absolute;
  top: 0;
  left: 50%;
  transform: translateX(-50%);
  width: 0;
  height: 100%;
  background-color: var(--color-purple);
  z-index: -1;
  transition: width 0.2s ease-in-out;
}
.btn_4:hover::after {
  width: 110%;
}