Jetpack's custom CSS errors

I use the TwentyTwelve theme as a basis for my design. I planned my site’s custom CSS in Firefox’s Stylish extension, where it worked perfectly. I copied the relevant code to Jetpack’s Custom CSS fields, but some of the rules just does not seem to apply.

Eg. I set header#masthead {display: flex;}, but it looks as it was originally defined by the theme, header {display: block;}. When I look at it in Firebug, it’s not that my code is overruled by the original, but that the custom code does not apply at all for some reason.

Site: https://chestofstories.com/