CSS in style section doesn't stick

I'm hoping someone can dumb this whole thing down for me. It seems really straightforward but I can not get any CSS I put in the Style section of the settings to work. Header and footer—no problem. But anything I put in for the style (even as simple as a background color) doesn't stick. It looks like all of the CSS from my site is loaded into the message and overrides any changes I make in the style section.

Hoping someone I can get help understanding this and getting it to work for me.

Thanks in advance for the help.