While this may be apparent to many in the know, it didn't occur to me until I'd used the heck out of this awesome feature.
Your handy Quick CSS field is a GLOBAL fix.
This means that if you set your header width to 1140px, it applies to all screens - tablets AND phones too! Not just your desktop screen, as many might be accustomed to editing.
Be careful when you use this and read your instructions carefully.