I guess every Magento 2 developer will run into this common problem at some point. You want to replace a static content file from the core, you copy it into your theme directory and make your changes, and in the end nothing happens, because Magento still serves the static file from the core. The first thing you can do in your development environment is to delete the deployed static content file. If Magento 2 is in developer mode, you can just make a new request to the frontend and the file will be automatically redeployed. If it is still not working, this can become a serious problem. In the next steps I want to explain how to debug the static file resolution to track down the issue.