make my pelipoyta look more like a real pelipoyta of ventti. rounded ocrners, minimaliistic, clean. put it in the center with atleast 50 percent width: <main>
      <div id="sisältö">
        <div id="pelipoyta">
          <div id="napit">
            <button id="ota-kortti">Ota kortti</button>
            <button id="jaa">Jää</button>
            <button id="uusi-peli">Uusi peli</button>
          </div>

          <div id="pelaajan-kortit">
            <div id="pelaaja1" class="kortti"></div>
            <div id="pelaaja2" class="kortti"></div>
            <div id="pelaaja3" class="kortti"></div>
            <div id="pelaaja4" class="kortti"></div>
            <div id="pelaaja5" class="kortti"></div>
          </div>

          <div id="tulos">Aloita peli painamalla "Uusi peli" -nappia.</div>

          <div id="jakajan-kortit">
            <div id="jakaja1" class="kortti"></div>
            <div id="jakaja2" class="kortti"></div>
            <div id="jakaja3" class="kortti"></div>
            <div id="jakaja4" class="kortti"></div>
            <div id="jakaja5" class="kortti"></div>
          </div>
        </div>
      </div>
    </main>css.* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  background-color: rgb(24, 24, 24);
  font-family: "Lato", sans-serif;
}

img {
  height: 1000px;
  padding-bottom: 50px;
  padding-top: 50px;
  display: flex;
  flex-direction: column;
  margin: 0 auto;
}

.container {
  max-width: 1200px;
  margin: 0 auto;
}

h1 {
  font-weight: 300;
}
h2 {
  margin: 32px 0;
  text-align: center;
  color: white;
}

h3 {
  color: white;
  margin: 20px 0;
  margin: 32px 0;
  text-align: center;
}

p {
  margin: 16px 0;
  line-height: 1.5;
  color: white;
}

/* Header */
header {
  text-align: center;
  background: url(https://images.unsplash.com/photo-1501003878151-d3cb87799705?ixlib=rb-4.0.3&ixid=MnwxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8&auto=format&fit=crop&w=1170&q=80)
    no-repeat fixed center center/cover;
}

header .container {
  padding: 48px;
}

header nav ul {
  list-style-type: none;
  margin-top: 32px;
}

header nav ul li {
  display: inline-block;
}

header nav ul li a,
.logo a {
  text-decoration: none;
  display: block;
  color: white;
  padding: 0 16px;
}

.logo a {
  font-size: 40px;
}

/* Hero Banner*/
#hero-banner {
  min-height: 500px;
}

#hero-banner h2 {
  font-size: 70px;
  color: white;
  text-align: center;
  padding-top: 150px;
}

#projects,
#about,
#skills {
  padding: 50px;
  min-height: 500px;
}

@media only screen and (max-width: 400px) {
  #hero-banner h2 {
    font-size: 300%;
    color: white;
    text-align: center;
    padding-top: 150px;
  }
}

.container {
  margin: 0 auto;
}

.content {
  text-align: center;
  padding-bottom: 2rem;
  color: coral;
  list-style-type: none;
}

/* Footer */
footer {
  background-color: rgb(0, 0, 0);
  text-align: center;
  color: rgb(255, 255, 255);
  font-size: 12px;
  padding: 8px;
}

footer form {
  margin: 0 auto;
  width: 60%;
  text-align: center;
}

footer form .form-group {
  margin: 2rem 0;
  display: flex;
}

footer form label {
  width: 200px;
  display: inline-block;
  text-align: left;
}

footer form .form-group input,
footer form .form-group textarea {
  padding: 0.5rem 1rem;
  border-radius: 5px;
  border: none;
  width: 60%;
  font-size: 1rem;
}

footer form .form-group textarea {
  min-height: 100px;
}

button {
  background-color: coral;
  text-transform: uppercase;
  border-radius: 5px;
  border: none;
  padding: 8px;
  color: #fff;
  font-size: 16px;
}

.sisältö button {
  background-color: coral;
  text-transform: uppercase;
  border-radius: 5px;
  border: none;
  padding: 10px 20px;
  color: #fff;
  font-size: 16px;
  margin-top: 20px;
}

#rekisteroidy {
  background-color: coral;
  text-transform: uppercase;
  border-radius: 5px;
  border: none;
  padding: 1rem 2rem;
  color: #fff;
  font-size: inherit;
}

.piilotettu {
  display: none;
}

.tehtävät {
  text-align: center;
  padding: 3rem 2rem;
}

hr {
  margin: auto;
  margin-bottom: 40px;
  margin-top: 40px;
  width: 70%;
}

#textstyle4 {
  display: none;
}

#kortit {
  display: grid;
  grid-template-columns: 100px 100px 100px;
  grid-template-rows: 120px 120px 120px;
  justify-content: center;
  align-items: center;
  padding-bottom: 20px;
}

.sisältö {
  text-align: center;
  padding-top: 10px;
  padding-bottom: 20px;
  display: flex;
  flex-direction: column;
  margin: 0 auto;
}

h3 {
  color: white;
  margin: 20px 0;
  text-align: center;
}

#valikko ul {
  list-style: none;
  width: 150px;
  margin: 0;
  padding: 5px;
  font-weight: bold;
  text-align: center;
}

#valikko a {
  text-decoration: none;
  display: inline-block;
  margin-top: 15px;
  margin-top: 15px;
}

#valikko ul li:not(:first-child) {
  display: none;
}

#valikko ul li:first-child {
  border: 3px solid white;
  border-radius: 10%; /* make the border circular */
  display: block;
  height: 30px;
  cursor: pointer;
  margin: 0 auto;
}

#valikko {
  width: 10%; /* or 90% */
  margin: 0 auto;
  margin-top: 20px;
  margin-bottom: 20px;
}

* {
  box-sizing: border-box;
}

#pelipoyta {
  background-color: rgba(21, 133, 21, 0.575);
  padding: 30px;
  border: 3px solid #000000;
  display: inline-block;
}

#napit {
  margin-bottom: 20px;
}

#uusi-peli {
  float: right;
}

#pelaajan-kortit,
#jakajan-kortit {
  display: flex;
}

#tulos {
  text-align: center;
  padding: 20px;
  font-size: 20px;
  font-weight: bold;
  height: 60px;
}

.kortti {
  display: inline-block;
  border: 1px solid black;
  padding: 5px;
  margin: 5px;
  width: 130px;
  height: 180px;
  text-align: center;
  line-height: 80px;
  font-size: 40px;
  font-family: Arial, sans-serif;
  border-radius: 10px;
}

.musta,
.punainen {
  background-color: #ffffff;
}

.kortti span {
  display: block;
}

.kortti .symboli {
  text-align: center;
  font-size: 30px;
}

.musta .symboli {
  color: #000000;
}

.punainen .symboli {
  color: #ff0000;
}

.kortti .tokanumero {
  -moz-transform: scale(-1, -1);
  -webkit-transform: scale(-1, -1);
  -o-transform: scale(-1, -1);
  -ms-transform: scale(-1, -1);
  transform: scale(-1, -1);
}