Your basic CSS is right. I would check to make sure:
1- Your css file is actually loading
I go to view source and look for my file name. If the file name appears in a giant weird JS object, but not in the css declarations that means the file can't be found (most likely a 404)
2- Your base theme isn't writing some overly specific CSS that you need to override.
Using the web inspector or a similar tool inspect the element you're trying to change, get your width where it should be to have your change apply (in that case greater than 1200 width) and see if there are styles overriding the one you wrote.
An easy temporary way to get around #2 would be to use !important at the end of your style, just to see if you're in the neighborhood, but that !important should be deleted of course :)