:root {
  --light-text-color: #f0f0f0;
  --gray-color: #6c757d;
}

* {
  font-family: "Roboto", sans-serif;
  padding: 0;
  margin: 0;
  color: var(--light-text-color);
}

html {
  overflow-y: overlay;
}

body {
  position: relative;
}

a {
  text-decoration: none;
}

a:hover {
  text-decoration: none;
}

label {
  margin-bottom: 0;
}

main#main {
  background-position: center;
  background-repeat: no-repeat;
  background-size: cover;

  height: 100vh;
  overflow: auto;
  position: relative;

  box-shadow: inset 2000px 2000px 2000px rgba(27, 20, 20, 0.8);
}

nav {
  background-color: rgba(27, 20, 20, 0.9);
  position: fixed !important;
  top: 0;
  left: 0;
  right: 0;

  z-index: 1000;
}

sup {
  position: unset;
  vertical-align: text-top;
}

#header {
  font-size: 50px;
  font-weight: 500;
  text-align: center;
  margin: 25px auto 40px;
}

#content {
  margin-top: 50px;
  overflow: auto;
}

/* style for footer  */
footer .nav-item {
  transition: 0.3s;
  transform: scale(1.1);
}

footer .nav-item i {
  transition: 0.5s;
  color: var(--gray-color);
}

footer .nav-item:hover {
  transform: scale(1.3);
  opacity: 1.2;
}

footer .nav-item:hover i {
  color: var(--light-text-color);
}

/* style scroll bar  */
::-webkit-scrollbar {
  width: 1.2rem;
  background: transparent;
}

::-webkit-scrollbar-button {
  height: 0.5rem;
}

::-webkit-scrollbar-track {
  width: 100%;
  background-color: transparent;
}

::-webkit-scrollbar-thumb {
  border-radius: 5rem;
  background: linear-gradient(to right, #999, #333);
  background-color: rgba(0, 0, 0, 0.6);

  border: 0.3rem solid transparent;
  background-clip: content-box;
}

/* style for header wrap in problems page and tools page  */
.header-wrap {
  display: flex;
  align-items: center;
  margin-bottom: 40px;
  width: 100%;
  justify-content: space-around;
}

.header-wrap .select-wrap {
  display: flex;
  align-items: center;
  justify-content: space-around;
}

.header-wrap form {
  display: flex;
  align-items: center;
  justify-content: space-around;
}

.header-wrap .title {
  min-width: 60px;
  margin-right: 12px;
  white-space: nowrap;
}

.header-wrap select,
.header-wrap .submit-btn,
.header-wrap .random-btn {
  padding: 10px 16px;
  background-color: transparent;
  border-radius: 5px;
  font-size: 16px;
  line-height: 16px;
}

.header-wrap .submit-btn {
  margin: 0 16px;
}

.header-wrap .submit-btn,
.header-wrap .random-btn {
  background-color: rgba(255, 255, 255, 0.1);
}

.header-wrap option {
  background-color: #231f22;
  padding: 4px;
}

@media (max-width: 768px) {
  .header-wrap form {
    flex-direction: column;
    align-items: center;
  }

  .header-wrap .submit-btn {
    margin: 16px;
  }
}