Generated by SassMeister.com.

input.scss

#menu {
  align-items: center;
  justify-content: center;
  display: flex;
}
.main-menu {
  width: 300px;
  padding: 0.5rem;
  border-radius: 0.5rem;
  border: 1px solid;
  background: white;
}
.main-menu .header {
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: horizontal;
  -webkit-box-direction: normal;
  -webkit-flex-direction: row;
  -ms-flex-direction: row;
  flex-direction: row;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
  -ms-flex-pack: center;
  justify-content: center;
  border-bottom: 1px solid;
}
.main-menu .header__title {
  font-weight: bold;
}
.main-menu .header__close {
  font-weight: bold;
  position: relative;
  left: 125px;
}
.main-menu .body {
  height: 300px;
  overflow-y: scroll;
}
.main-menu .message_userId {
  color: blue;
}
.main-menu .message_userId:after {
  content: ":";
}
.main-menu .form {
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: horizontal;
  -webkit-box-direction: normal;
  -webkit-flex-direction: row;
  -ms-flex-direction: row;
  flex-direction: row;
  -webkit-box-align: center;
  -webkit-align-items: center;
  -ms-flex-align: center;
  align-items: center;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
  -ms-flex-pack: center;
  justify-content: center;
}
.main-menu .input-text {
  width: 200px;
}
.main-menu .announce {
  height: 200px;
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
  -ms-flex-pack: center;
  justify-content: center;
  -webkit-box-align: center;
  -webkit-align-items: center;
  -ms-flex-align: center;
  align-items: center;
}

.room-list {
  margin-top: 1rem;
  background: #33333320;
  border-radius: 10px;
  padding: 1rem 0.5rem 1rem 0.5rem;
  display:flex;
  flex-direction: column;
  justify-content:space-between;
  opacity: 0.9;
  .list-item {
    border-bottom: 1px solid #333;
    padding-top: 0.5rem;
    .name {
    }

    .join-button {
    }
  }
}

markup.html

<div class="main-menu animate__animated animate__fadeIn">
    <div class="header"><span class="header__title">対戦を始める</span><span class="header__close">×</span></div>
    <div class="body">
        <div class="single-play">
            <p> シングルプレイ - Single Play</p>
            <p>（準備中 - Coming Soon）</p><input disabled="" type="button" value="CPUと対戦" class="create">
        </div>
        <div class="multi-play">
            <p>マルチプレイ - Multi Play</p>
            <div class="crete-room">
                <div>新規に作る - Create Room</div><input type="text" name="createRoomName" class="name"><input type="button"
                    value="作る" class="create">
            </div>
            <p>ルーム一覧</p>
            <div class="room-list">
                <div class="list-item">
                    <span class="name">AggressiveMule</span>
                    <span class="join-button">参加</span>
                </div>
                        <div class="list-item">
                    <span class="name">AggressiveMule</span>
                    <span class="join-button">参加</span>
                </div>
            </div>
            <div class="join-room">
                <div>友達のところに入る - Join Room</div><input type="text" name="joinRoomName" class="name"><input type="button"
                    value="参加" class="join">
            </div>
        </div>
    </div>
</div>

output.css

#menu {
  -webkit-box-align: center;
  -webkit-align-items: center;
      -ms-flex-align: center;
          align-items: center;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
      -ms-flex-pack: center;
          justify-content: center;
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
}

.main-menu {
  width: 300px;
  padding: 0.5rem;
  border-radius: 0.5rem;
  border: 1px solid;
  background: white;
}

.main-menu .header {
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: horizontal;
  -webkit-box-direction: normal;
  -webkit-flex-direction: row;
  -ms-flex-direction: row;
  flex-direction: row;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
  -ms-flex-pack: center;
  justify-content: center;
  border-bottom: 1px solid;
}

.main-menu .header__title {
  font-weight: bold;
}

.main-menu .header__close {
  font-weight: bold;
  position: relative;
  left: 125px;
}

.main-menu .body {
  height: 300px;
  overflow-y: scroll;
}

.main-menu .message_userId {
  color: blue;
}

.main-menu .message_userId:after {
  content: ":";
}

.main-menu .form {
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: horizontal;
  -webkit-box-direction: normal;
  -webkit-flex-direction: row;
  -ms-flex-direction: row;
  flex-direction: row;
  -webkit-box-align: center;
  -webkit-align-items: center;
  -ms-flex-align: center;
  align-items: center;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
  -ms-flex-pack: center;
  justify-content: center;
}

.main-menu .input-text {
  width: 200px;
}

.main-menu .announce {
  height: 200px;
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
  -ms-flex-pack: center;
  justify-content: center;
  -webkit-box-align: center;
  -webkit-align-items: center;
  -ms-flex-align: center;
  align-items: center;
}

.room-list {
  margin-top: 1rem;
  background: #33333320;
  border-radius: 10px;
  padding: 1rem 0.5rem 1rem 0.5rem;
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: vertical;
  -webkit-box-direction: normal;
  -webkit-flex-direction: column;
      -ms-flex-direction: column;
          flex-direction: column;
  -webkit-box-pack: justify;
  -webkit-justify-content: space-between;
      -ms-flex-pack: justify;
          justify-content: space-between;
  opacity: 0.9;
}
.room-list .list-item {
  border-bottom: 1px solid #333;
  padding-top: 0.5rem;
}

settings.json

{
  "sass": {
    "compiler": "dart-sass/1.32.12",
    "extensions": {},
    "syntax": "SCSS",
    "outputStyle": "expanded"
  },
  "autoprefixer": true
}