With Chrome 65 this has become trivial.
Using local overrides – Chrome 65
What is it?
It is a new feature that allows us to override a websites code/css with a local copy that is persisted across sessions. Once you override a file it shall remain until you remove the override.
How to set it up?
- Open the Sources panel.
- Open the Overrides tab.
Open overrides tab - Click Setup Overrides.
- Select which directory you want to save your changes to.
- At the top of your viewport, click Allow to give DevTools read and write access to the directory.
- Make your changes.
After you add a folder you can switch to the network tab and right click on any file and select “Save for overrides”. I have already overridden scripts.js so it shows with a “blue dot”.