The custom stylesheet in the customizer is applied to every page, so that'll work (and has live preview).
Adding it to your styles.css file in the child theme will also work (and be applied to all pages on the frontend) and is the best long term option.
If you've added a rule, but it's not taking effect, you like have a
selector specificity issue -- i.e. the theme is overriding your custom style.
p{} will be applied to all pragraph elements, but anything more specific will override it.
body p{} is more specific.
For 2016 you can target just p tags in the main content area:
body .site-content p{}
OR
body #content p{}
As IDs (the #) are more specific than classes.
And so on... (let's just avoid !important as it generally is more trouble than it's worth).
It may also be caching (shift + refresh can help in this case).
That should get you where you want to go... good luck!
Nick Ciske
CTO @ LuminFire