style.css

body{
    background: #536976;  /* fallback for old browsers */
    background: -webkit-linear-gradient(to right, #292E49, #536976);  /* Chrome 10-25, Safari 5.1-6 */
    background: linear-gradient(to right, #292E49, #536976); /* W3C, IE 10+/ Edge, Firefox 16+, Chrome 26+, Opera 12+, Safari 7+ */
    color:hsl(239, 57%, 33%,10);
    font-weight:540;
    padding: 40px;
    font-family: sans-serif;
   font-size: larger;
   margin-top: 80px;
   width: fit-content;
   margin-left: auto;
   margin-right: auto;
   padding: 10px 40px 10px;
}
h1{
    box-sizing: border-box;
  background-color: hsl(209, 52%, 31%,0.5);
  color: wheat;
    border-top:6px solid #022140;
    border-top-left-radius: 50px;
    border-top-right-radius: 50px;
    margin-block-start: 0em;
    margin-block-end: 0em;


}


.calculator{
display: flex;
flex-direction: column;
text-align:center;
}

.child-container{
    background-color: hsl(0, 0%, 96%,0.5);
    padding: 20px 0px 20px;
 
}
.child-container2{
    background: #B993D6;  /* fallback for old browsers */
    background-color: hsl(0, 0%, 96%,0.5);
    height:max-content;
    border-bottom-left-radius:50px ;
    border-bottom-right-radius:50px ;
    border-bottom:6px solid #022140;
    padding-bottom: 20px;
}

.container{
    
    padding: 20px 0px 20px;
}
.billTip{
    /*box-sizing: border-box;*/
    padding: 0 50px;
}
#calcbtn{
    padding: 10px;
    background-color: hsl(209, 52%, 31%);
    font-size: large;
    color: whitesmoke;
    border-radius: 10px;
    border: 2px solid hsl(209, 52%, 31%);
}
.btn{
  
      background-color: hsl(0, 0%, 96%,0.5);
}

.calcinput{
    width: 150px;
    height: 25px;
    border-radius: 5px;
    border: 2px solid hsl(209, 52%, 31%);
    color: black;
    font-weight: bold;
    font-size: 15px;
   
 }
 .calc {
    width: 120px;
    height: 25px;
    border-radius: 5px;
    border: 2px solid hsl(209, 52%, 31%);
    color: black;
    font-weight: bold;
    font-size: 15px;
 }



/* For mobile phones: */
[class*="col-"] {
    width: 100%;
  }
  
  @media only screen and (min-width: 600px) {
    /* For tablets: */   
    .col-s-12 {width: 100%;}
  }
  
  @media only screen and (min-width: 1000px) {
    /* For desktop: */
    .col-12 {width: 100%;}
  }