You need a Browser extention that allow you to inject css into a page.
I use and can recommend [this one](https://chrome.google.com/webstore/detail/user-javascript-and-css/nbhcbdghjpllgmfilhnhkllmkecfmpld) for Chrome

1. Install/Enable Extention
2. Go to any mdn article
3. Click Extention and "Add Rule"
4. Enter the css posted above
5. Save, (close the extention tab), then reload the MDN page
6. ???
7. Profit!