if you want to insert your own CSS style in any webpage in custom webview then you can use this code in Evaluate Js
var divNode = document.createElement("div");
divNode.innerHTML = "<br><style>h1 { color:Red; margin:16px;}</style>";
document.body.appendChild(divNode);
just write your css code between <style>
and </style>
you can hide any component or element by the property display:none;