estilos.css

/* configurações gerais da página */
body {
  margin: 0;
  background: #112 url(images/stars2.jpg) repeat;
  background-size: 75%;
  font-family: "Lato", sans-serif;
  color: #fff;
  font-size: 18px;
}

p {
  text-align: justify;
}

h1, h2, h3, h4, h5, h6 {
  margin: 0.25em;
  font-family: "Space Mono", monospace;
}

/* fundo de estrelas esmaecendo */
body::before,
body::after {
  content: " ";
  display:block;
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  z-index: -1;
  background: transparent url(images/twinkling.png) repeat;
  animation: move-twink-back 500s linear infinite;
}

body::after {
  animation-direction: reverse;
}

@keyframes move-twink-back {
  from {  background-position: 0 0; }
  to   {  background-position: -10000px 5000px; }
}

/* muda a cor da seleção para um azul bacana */
*::selection {
  background-color: #6dacff;
}

/* layout do cabeçalho da página */
header {
  background-color: rgba(255, 255, 255, .1);
  padding: 0 1em;
  display: flex;
  flex-direction: row;
  justify-content: space-between;
  animation: chegando-de-cima 700ms cubic-bezier(0.18, 0.62, 0.22, 1) 0ms 1 forwards;
}

header * {
  transform: translate3d(0,-100%, 0);
  animation: chegando-de-cima 700ms cubic-bezier(0.18, 0.62, 0.22, 1) 200ms 1 forwards;
}

header :nth-child(2n) {
  animation-delay: 200ms;
}

header :nth-child(2n+1) {
  animation-delay: 400ms;
}

header ul :nth-child(3n+1) {
  animation-delay: 600ms;
}

header ul :nth-child(3n+2) {
  animation-delay: 800ms;
}

header ul :nth-child(3n) {
  animation-delay: 1000ms;
}

@keyframes chegando-de-cima {
  from  {
    opacity: 0.2;
    transform: translate3d(0, -100%, 0);
  }
  to  {
    opacity: 1;
    transform: translate3d(0, 0, 0);
  }
}

/* estilizando o menu superior */
#logo {
  float: left;
  height: 1.25em;
  margin-right: 0.5em;
}

#menu-principal ul {
  display: flex;
  flex-direction: row;
  height: 100%;
  margin: 0;
  padding: 0;
  border-left: 1px solid rgba(0, 0, 0, .2);
  border-right: 1px solid rgba(255, 255, 255, .1);
}

#menu-principal li {
  display: inline-block;
  list-style-type: none;
  list-style-image: none;
  margin: 0;
  border-left: 1px solid rgba(255, 255, 255, .1);
  border-right: 1px solid rgba(0, 0, 0, .2);
}

#menu-principal a {
  position: relative;
  display: inline-block;
  box-sizing: border-box;
  width: 100%;
  height: 100%;
  padding: 2em 1.5em 0.75em;
  color: currentColor;
  text-decoration: none;
}

#menu-principal a::before {
  content: " ";
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
  opacity: 0.05;
  transition: opacity 200ms ease;
}

#menu-principal a:hover::before {
  opacity: 0.25;
}

#menu-principal li:nth-child(1) a::before {
  background-color: #128fff;
}

#menu-principal li:nth-child(2) a::before {
  background-color: #d012ff;
}

#menu-principal li:nth-child(3) a::before {
  background-color: #12ff59;
}

/* layout do conteúdo principal da página */
main {
  padding: 1em;
}

.aba-conteudo {
  display: none;
}

.aba-conteudo.ativa {
  display: block;
}

/* estilos do exercício da gravitação */
#gravitacao .colunas {
  display: flex;
  flex-direction: row;
  gap: 3em;
}

#gravitacao .coluna {
  flex: 1;
}

#gravitacao input {
  width: 6em;
}

.controles {
  display: flex;
  flex-direction: row;
  gap: 1em;
  margin-bottom: 1em;
}

/* estilos do exercício dos aliens */
#aliens p {
  white-space: nowrap;
  height: 1.2em;
  font-size: 1em;
  line-height: 1.2em;
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
}

#aliens p.expandido {
  white-space: normal;
  height: auto;
}

.botao-expandir-retrair {
  box-sizing: border-box;
  height: 1em;
  width: 1em;
  padding: 0;
  margin: 0 .5em 0 0;
  font-size: inherit;
  line-height: 1em;
  border-width: 1px;
}

/* estilos do exercício philae */
#galeria {
  margin: 0 auto;
  max-width: 700px;
}

#slide {
  aspect-ratio: auto 739 / 415;
  border: 1px solid gray;
  display: block;
  max-width: 100%;
}

.navegar {
  width: 150px;
  padding: 0.5em 0.5em;
  margin-top: 2px;
  font-size: 1em;
  border: 2px solid currentColor;
  background-color: transparent;
  color: #999;
  transition: all 100ms ease;
}

.navegar:hover {
  color: white;
  cursor: pointer;
}

.navegar:active {
  text-indent: 2px;
}

#anterior {
  float: left;
  border-radius: 0 0 0 10px;
}

#proximo {
  float: right;
  border-radius: 0 0 10px 0;
}

#anterior:hover {
  transform: translateX(-5px);
}

#proximo:hover {
  transform: translateX(5px);
}