Replacing the layout, styles and java-scripts of the site with your own?

Question

HTML

There was a task to replace the existing design of the resource with our own, because. The current one doesn't suit me. To begin with, I want to do it at the level of my browser, then push it all into a chrome extension (in the future it is possible for other browsers).
Tell me which way to dig, starting with a suitable browser extension and then compiling into an extension.
Ideally, I want the design to be pulled from my computer / extension and not once again pull the server, such as loading the original design and only then replacing it with my own. Layout skills are available.

Answer 1

https://addons.mozilla.org/en/firefox/addon/stylish/
https://chrome.google.com/webstore/detail/stylish/...
And a lot of analogues

Answer 2

In IE\browser settings there is an item "enable custom css". For famous sites like VK, there are many ready-made themes, but there are no such resources for unknown resources.
You can find themes that try to adapt to all sites, but there is no guarantee that they will work correctly.
And of course, from the first point follows the opportunity to create a theme for yourself.