怎么让内容适应屏幕大小啊

如何自适应屏幕大小，比如我在一个大一点的屏幕页面打开或者手机打开就变了
求帮忙

img


@charset "utf-8";

html, body {
  height: 100%;
  width: 100%;
  position: relative;
  transition: .2s;
    margin:0;
  padding:0;
  background-color: #EFE2E2;
}

header {
  display: flex;
  box-sizing: border-box;
  flex-direction: row;
  justify-content: flex-end;
  align-items: center;
  background-color: #C8AAAA;
  font-size: 0.8rem;
  font-family: monospace;
}


header > img {
  margin: 10px auto 10px 10px;
}


header > nav > ul > li {
  display: inline-block;
  list-style-type: none;
  padding: 0 1.252rem;
}


header > nav > ul > li > a {
    text-decoration: none;
    color: #333;
}


header > nav > ul > li > a:hover {
    color: white;
}



.titleimg{
    background-image: url("perth view.jpg");
    height: 100vh;
    background-repeat: no-repeat;
    background-size: 1540px;
    background-attachment: fixed;
    width: 100%;
    transition: 0.5s ease 0s;
    animation: load 0.5s; 
    justify-content: flex-end;
}

.titleimg .title{
  text-align: center;
    width: 1540px;
    padding:200px;
    font-size: 100px;
    font-weight: bolder;
    color: floralwhite;
  
}

.main{
    scroll-snap-type: y mandatory;
    overflow: scroll;
    overflow-x: hidden;
    height: 100vh;
}

.sec{
    height: 100vh;
    width: 100vw;
    scroll-snap-align: start;
}

.pt{
    display: flex;
    flex-direction: column;
    height: 400px;
    width: 250px;
    float: left;
    margin: 150px 0px 0px 105px;
    background: rgba(219, 235, 241,0.8);
    box-shadow: 0 0 15px rgb(59, 73, 95);
    border-radius: 15px;
    overflow: hidden;
    scroll-snap-align: start;
    transition: all 0.5s ease 0s;
}
.pt:hover{
    cursor: pointer;
    transform: scale(1.05);
}

.imghid{
    overflow: hidden;
}
.part1 .img{
    background-image: url("quokka.jpg");
}
.part2 .img{
    background-image: url("kings park.png");
}
.part3 .img{
    background-image: url("swan river.png");
}

.pt .img{
    height: 190px;
    width: 250px;
    background-size: cover;
    background-position: 0px -30px;
    transition: all 0.5s ease 0s;
    opacity: 0.8;
}

.pt .img:hover{
    transform: scale(1.1);
}

.pt .txt{
    height: 310px;
    width: 200px;
    overflow: hidden;
    display: flex;
    flex-direction: column;
    margin: auto;
    
}

.pt .txt h1{
    text-align: center;
    font-size: 3em;
    color: rgb(28, 44, 68);
    height: 30px;
    line-height: 30px;
}

.pt .txt p{
    margin-top: -2px;
  text-align: center;
  font-family:Lucida Sans;
    word-wrap: break-word;
    color: rgb(114, 182, 202);
}
@keyframes load{
    0%{
        background-position-y: 50px;
        opacity: 0;
    }
    100%{
        background-position-y: 0px;
        opacity: 1;
    }
}