Here I have a multi-select list which looks like below.
But I am unable to change the CSS. this is the code so far I have tried, I am able to get outline colors, inside ticks am unable to see it.
input[type=checkbox], input[type=radio] {
    margin: 4px 0 0;
    margin-top: 1px\9;
    height: 16px;
    border-radius: 2px;
    line-height: normal;
    box-shadow: none;
    outline-offset: -1px;
    -webkit-appearance: none;
    width: 16px !important;
    margin: 5px;
     appearance:none;
     outline: 2px solid #455eab;
     font-size: 2em;
    
}
.form-check-input{
    width: 35px;
}
input[type="checkbox"]:checked ~ label::before {
    color: #455eab;
}  <input class="form-check-input"[checked]=true type="checkbox">
  <input class="form-check-input" type="checkbox" >
  <input class="form-check-input" type="checkbox" >
  <input class="form-check-input" type="checkbox" >
 
    