style.css

@import url("https://fonts.googleapis.com/css2?family=Saira:wght@400;500&display=swap");

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

body {
  background: rgb(2, 0, 36);
  background: linear-gradient(
    90deg,
    rgba(2, 0, 36, 1) 0%,
    rgba(9, 9, 121, 1) 35%,
    rgba(0, 212, 255, 1) 100%
  );
  font-family: "Saira", sans-serif;
  color: #f5f5f5;
  display: flex;
  align-items: center;
  height: 100vh;
}

h1,
label {
  text-shadow: 1px 1px 1px black;
  font-weight: 400;
}

/* conteúdo principal */
#container {
  width: 55%;
  margin: auto;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  gap: 25px;
  background-color: rgba(255, 255, 255, 0.2);
  padding: 50px;
  border-radius: 6px;
}

h1 {
  font-size: 2.5rem;
}

form {
  width: 70%;
  display: flex;
  flex-direction: column;
  gap: 10px;
}

label {
  font-size: 1.6rem;
  margin-top: 5px;
}

input[type="text"]:valid,
input[type="email"]:valid,
textarea:valid {
  border-bottom: 3px solid #23b46b;
}

input[type="text"]:focus:invalid,
input[type="email"]:focus:invalid,
textarea:focus:invalid {
  border-bottom: 3px solid #b42325;
}

.contact-form,
textarea {
  border-radius: 3px 3px 0 0;
  border: none;
  background-color: rgba(255, 255, 255, 0.4);
  color: #000000;
  border-bottom: 3px solid #f5f5f5;
  font-family: Arial, Helvetica, sans-serif;
  font-size: 1.4rem;
}

.contact-form {
  height: 45px;
  padding: 5px 5px 5px 10px;
}

textarea {
  resize: none;
  font-size: 1.4rem;
  padding: 10px;
}

.contact-form,
textarea:focus {
  outline: none;
}

.contact-form::placeholder,
textarea::placeholder {
  color: #515151;
}

.btn {
  margin-top: 40px;
  width: 20%;
  height: 55px;
  color: inherit;
  font-size: 1.3rem;
  background-color: rgba(255, 255, 255, 0.5);
  color: inherit;
  border: none;
  border-radius: 3px;
  text-shadow: 0.5px 0.5px 0.5px rgba(0, 0, 0, 0.9);
  display: flex;
  align-self: flex-start;
  justify-content: center;
  cursor: pointer;
  transition: transform 0.5s;
  box-shadow: 0.7px 0.7px 4px rgba(0, 0, 0, 0.9);
}

.btn:hover {
  transform: translateY(-3px);
  background-color: rgba(255, 255, 255, 0.4);
}