how to apply css on iframe code in css, javascript and jquery, html?

I’ve try this code for Iframe code css but it dosen’t working… can you suggest me how to apply css for iframe..

window.onload = function() { let myiFrame = document.getElementById("myiFrame"); let doc = myiFrame.contentDocument; doc.body.innerHTML = doc.body.innerHTML + '<style>.form_generater_form_div .block_label {font-weight: bold !important;}</style>'; }

OR

$("#myiFrame").on("load", function() { let head = $("#myiFrame").contents().find("head"); let css = '<style>.form_generater_form_div .block_label {font-weight: bold !important;}</style>'; $(head).append(css); });