請問各位大大，
我做了一個網站，然後我想要用JS判斷：
如果用戶使用的裝置是行動裝置（手機、平板等），
則改變背景的顏色和讓所有東西隱藏（我的預想是用display: none），
我目前已經寫好了判斷裝置的JS（其實是在網路上找的），
但不知道要怎麼用JS改變背景顏色和隱藏所有東西，
請問各位大大有什麼方法嗎？

HTML程式碼（body內）：

  <nav class="nav"> 
              <a class="bar_a" id="bar_a1" aria-current="page" target='_self' href="index.html">部落格</a>
              <a class="bar_a" id="bar_a2" aria-current="page" target='_self' href="../服務/indexService.html">服務</a>
              <a class="bar_a" id="bar_a3" aria-current="page" target='_self' href="../關於/indexAbout.html">關於Inspi.</a>
            </div>
        </nav>
        


          <img src="https://i0.zi.org.tw/kocpc/2021/07/1625708401-00890058e5109063effb7c201d1c1345.jpg" id="img20210813" class="all20210813">
          <div id="fixing">此頁面尚在開發中，敬請期待！</div>

CSS程式碼：

html, body, div, span, applet, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
a, abbr, acronym, address, big, cite, code,
del, dfn, em, img, ins, kbd, q, s, samp,
small, strike, strong, sub, sup, tt, var,
b, u, i, center,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td,
article, aside, canvas, details, embed,
figure, figcaption, footer, header, hgroup,
menu, nav, output, ruby, section, summary,
time, mark, audio, video {
margin: 0;
padding: 0;
border: 0;
font-size: 100%;
font: inherit;
vertical-align: baseline;
}
/* HTML5 display-role reset for older browsers */
article, aside, details, figcaption, figure,
footer, header, hgroup, menu, nav, section {
display: block;
}
body {
line-height: 1;
}
ol, ul {
list-style: none;
}
blockquote, q {
quotes: none;
}
blockquote:before, blockquote:after,
q:before, q:after {
content: '';
content: none;
}
table {
border-collapse: collapse;
border-spacing: 0;
}





/***WhenItLoadingh***/
.loading{
  cursor: wait;
}




/***操作區***/
img{
  user-select: none;
  -webkit-user-drag: none;
}
.ifMoble{
  background-color: gray;
}
.nav {
  background: rgb(120, 155, 103);
  display: flex;
  align-items: center;
  user-select: none;
  -webkit-user-drag: none;
  height: 90px;
}
.bar_a {
  color: #fff;
  text-decoration: none;
  font-family: Arial, Helvetica, sans-serif;
  font-weight: 400;
  font-size: 19px;
  margin: 0 24px;
  font-size: 33px;
  padding: 9px 24px;
  border: 8px solid transparent;
  border-radius: 32px;
  transition: .13s;
  position: relative;
  left: 5%;
  user-select: none;
  -webkit-user-drag: none;
}
.bar_a#bar_a1 {
  border: 8px solid rgb(156, 197, 138);
  
}
.bar_a#bar_a1:hover {
  border-color: rgb(169, 212, 149);
  transition: .13s;
}
.bar_a:hover {
  border-color: rgb(133, 172, 117);
}

/***部落格***/
/***2021 08 13***/
#img20210813{
    width: 300px;
    height: 200px;
    border-radius: 10px;
    position: fixed;
    top: 160px;
    left: 60px;
    vertical-align: text-top;

    display: none;
}
#word20210813{
    position: relative;
    bottom: 20%;
    left: 50%;
}
#fixing{
  font-size: 70px;
  font-family: Arial, Helvetica, sans-serif;
  font-weight: 400;
  color: rgb(133, 172, 117);
  text-align: center;
  justify-content: center;
}

JS程式碼：

window.addEventListener('load', function(){
  $('html').toggleClass('loading')
});
function detectmob() { 
  if( navigator.userAgent.match(/Android/i)
  || navigator.userAgent.match(/webOS/i)
  || navigator.userAgent.match(/iPhone/i)
  || navigator.userAgent.match(/iPad/i)
  || navigator.userAgent.match(/iPod/i)
  || navigator.userAgent.match(/BlackBerry/i)
  || navigator.userAgent.match(/Windows Phone/i)
  ){
    return true;
   }
  else {
     return false;
   }
 }