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:root {
  --main-bg-color: white;
  --text-color: #9932cc;
}

body {
  background-color: white;
}

.modal__window {
  background-color: white;
  position: fixed;
  width: 380px;
  margin: auto;
  left: 0;
  right: 0;
  top: 20%;
  font-family: "Rubik", sans-serif;
  box-shadow: 0 3px 7px rgba(0, 0, 0, 0.5);
}
.modal__window .modal__window__label {
  height: 70px;
  text-align: center;
  padding-top: 25px;
  background-color: var(--main-bg-color);
  color: rgba(0, 0, 0, 0.719);
  font-size: 23px;
  text-transform: uppercase;
  position: relative;
  padding-bottom: 15px;
  border-bottom: 1.4px solid var(--text-color);
}
.modal__window__form {
  margin-top: 35px;
}
.modal__window .modal__window__form .modal__window__form__object {
  height: 65px;
  font-size: 15px;
  margin: 8px auto;
  display: flex;
  flex-flow: column;
  width: 80%;
}

.modal__window .modal__window__form .modal__window__form__object input {
  height: 45px;
  padding-left: 10px;
  font-size: 17px;
  border: 1px solid #cccccc;
  outline: none;
  border-radius: 1px;
  background-color: #00000007;
}

.modal__window .modal__window__form .modal__window__form__button {
  text-align: center;
}
.modal__window .modal__window__form .modal__window__form__button {
  left: 0;
  width: 200px;
  height: 45px;
  display: flex;
  text-align: center;
  margin: auto;
  margin-bottom: 25px;
  /* position: relative;

  box-sizing: border-box;
  background: white;
  background-clip: padding-box;
  border: solid 3px transparent;
  border-radius: 1em;*/
}

.modal__window .modal__window__form .modal__window__form__button button {
  text-transform: uppercase;
  font-family: "Rubik", sans-serif;
  background-color: white;
  color: var(--text-color);
  border: none;
  border: 1.4px solid var(--text-color);
  border-radius: 4px;
  /* border-radius: inherit; !importanté */
  height: 45px;
  width: 200px;
  outline: none;
  font-size: 15px;
  /* background: linear-gradient(to right, #f6d365 0%, #fda085 51%, #f6d365 100%);
  display: table;
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent; */
}

/* .modal__window .modal__window__form .modal__window__form__button::before {
  content: "";
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  z-index: -1;
  margin: -3px;
  border-radius: inherit;
  background: linear-gradient(to right, #f6d365 0%, #fda085 51%, #f6d365 100%);
} */

.modal__window
  .modal__window__form
  .modal__window__form__object
  .modal__form__error {
  height: 15px;
  font-size: 13px;
  color: red;
  text-align: left;
  display: none;
}

.modal__success {
  position: fixed;
  left: 0;
  right: 0;
  margin: auto;
  text-align: center;
  width: 19%;
  padding: 15px 25px;
  background-color: white;
  font-size: 15px;
  border: 1px solid black;
  box-shadow: 0 3px 7px rgba(0, 0, 0, 0.5);
  font-family: "Rubik", sans-serif;
}
.text__button button {
  font-size: 20px;
  margin: 30px;
  border: none;
  text-transform: lowercase;
  font-family: Rubik;
  color: black;
  padding: 15px 25px;
  background-color: #ffd200;
  box-shadow: 1px 1px #dab200;
  outline: none;
  -webkit-transition: 0.5s ease; /* Safari and Chrome */
  -moz-transition: 0.5s ease; /* Firefox 4 */
  -o-transition: 0.5s ease; /* Opera */
}

.text__button button:hover {
  box-shadow: 1px 1px #dab200, 2px 2px #dab200, 3px 3px #dab200, 4px 4px #dab200;
  -webkit-transform: translateX(-3px);
  transform: translateX(-3px);
  -webkit-transition: all 0.5s ease;
  -moz-transition: all 0.5s ease;
  -o-transition: all 0.5s ease;
  transition: all 0.5s ease;
}

let error = false;

const regNumber = /^((\+7|7|8)+([0-9]){10})$/gm;
const regEmail = /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
const errorMessage = document.getElementsByClassName("modal__form__error");

function send(event) {
  event.preventDefault();
  let firstNumberError = false;
  let firstEmailError = false;
  const email = document.getElementsByClassName("modal__email");
  const number = document.getElementsByClassName("modal__number");

  const label = document.getElementsByClassName("modal__window__label");
  label[0].style.setProperty("--main-bg-color", "white");

  if (number[0].value === "") {
    errorMessage[0].style.display = "block";
    number[0].style.border = "1px solid red";
    error = true;
    firstNumberError = true;
  }
  if (email[0].value === "") {
    errorMessage[2].style.display = "block";
    email[0].style.border = "1px solid red";
    error = true;
    firstEmailError = true;
  }
  if (number[0].value.match(regNumber) === null && !firstNumberError) {
    errorMessage[1].style.display = "block";
    number[0].style.border = "1px solid red";
    error = true;
  }
  if (email[0].value.match(regEmail) === null && !firstEmailError) {
    errorMessage[3].style.display = "block";
    email[0].style.border = "1px solid red";
    error = true;
  }
  if (!error) {
    console.log("123");
  }
}

document.getElementById("modal__form").addEventListener("submit", send);

const onChangeNumber = document.getElementById("modal__form__error__number");

onChangeNumber.addEventListener("input", event => {
  errorMessage[1].style.display = "none";
  if (onChangeNumber.value !== "") {
    errorMessage[0].style.display = "none";
    onChangeNumber.style.border = "1px solid #cccccc";
    error = false;
  }
});

const onChangeEmail = document.getElementById("modal__form__error__email");

onChangeEmail.addEventListener("input", event => {
  errorMessage[3].style.display = "none";
  if (onChangeEmail.value !== "") {
    errorMessage[2].style.display = "none";
    onChangeEmail.style.border = "1px solid #cccccc";
    error = false;
  }
});
<!DOCTYPE html>
<html>
  <head>
    <title>Parcel Sandbox</title>
    <meta charset="UTF-8" />
    <link
      rel="stylesheet"
      href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css"
      integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T"
      crossorigin="anonymous"
    />
    <link
      href="https://fonts.googleapis.com/css?family=Rubik&display=swap"
      rel="stylesheet"
    />
    <link rel="stylesheet" href="./style.css" />
    <script
      src="https://code.jquery.com/jquery-3.4.1.js"
      integrity="sha256-WpOohJOqMqqyKL9FccASB9O0KwACQJpFTUBLTYOVvVU="
      crossorigin="anonymous"
    ></script>
    <script src="index.js"></script>
  </head>

  <body>
    <div class="modal__window">
      <div class="modal__window__label">
        <span>Оставьте вашу заявку</span>
      </div>
      <div class="modal__window__form">
        <form action=" " id="modal__form">
          <div class="modal__window__form__object">
            <input
              autocomplete="off"
              type="text"
              id="name"
              name="user_name"
              placeholder="Введите ваше имя"
            />
          </div>
          <div class="modal__window__form__object">
            <input
              autocomplete="off"
              class="modal__number"
              type="text"
              id="modal__form__error__number"
              name="user_number"
              placeholder="Введите ваш телефон"
            />
            <div class="modal__form__error">
              <span>Поле обязательно для заполнениия</span>
            </div>
            <div class="modal__form__error">
              <span>Проверьте правильность веденного номера</span>
            </div>
          </div>
          <div class="modal__window__form__object">
            <input
              autocomplete="off"
              class="modal__email"
              type="text"
              id="modal__form__error__email"
              name="user_mail"
              placeholder="Введите ваш email"
            />
            <div class="modal__form__error">
              <span>Поле обязательно для заполнениия</span>
            </div>
            <div class="modal__form__error">
              <span>Проверьте правильность веденного email</span>
            </div>
          </div>

          <div class="modal__window__form__button">
            <button type="submit" class="modal__window__form__thebutton">
              Отправить заявку
            </button>
          </div>
        </form>
      </div>
    </div>

    <div class="modal__success">
      <div class="modal_success__container">
        <span>ВАША ЗАЯВКА ПРИНЯТА</span>
      </div>
    </div>

    <div class="text__button">
      <button>
        обратная связь
      </button>
    </div>
  </body>
</html>