I recently created Boundary, a CSS+JS library to solve problems just like this. Boundary creates elements that are completely separate from the existing webpage's CSS.
Take creating a dialog for example. After installing Boundary, you can do this in your content script
var dialog = Boundary.createBox("yourDialogID", "yourDialogClassName");
Boundary.loadBoxCSS("#yourDialogID", "style-for-elems-in-dialog.css");
Boundary.appendToBox(
"#yourDialogID",
"<button id='submit_button'>submit</button>"
);
Boundary.find("#submit_button").click(function() {
// find() function returns a regular jQuery DOM element
// so you can do whatever you want with it.
// some js after button is clicked.
});
Elements within #yourDialogID will not be affected by the existing webpage.
Hope this helps. Please let me know if you have any question.
https://github.com/liviavinci/Boundary