You'll need to change the HTML which is embedded in the page.
The search form html is :
<form id="searchform" class="" role="search" action="https://www.cgway.net/" method="get"><input id="s" name="s" type="text" value="" placeholder="ماذا تريد ان تتعلم ؟"></form>
Next you can add this CSS :
form#searchform {
max-width: 80vw;
width: 600px;
border: none;
}
input#s {
text-align: center;
background: rgba(0,0,0,0.5);
border-color: rgba(0,0,0,0.1);
color: #fff;
font-size: 24px;
padding: 12px;
border-radius: 5px;
}
div#slide-1-layer-11 {
border: none;
}
input#s::placeholder {
color: #fff;
}
result :
http://prntscr.com/n1xftl