Need some help with twenty fifteen font sizes

I am no css expert that’s for sure ??

I am using the default theme twenty fifteen. I want to make a few small adjustments but the css is driving me crazy, the responsiveness of the fonts is making it too complicated for me ??

I basically using pages not posts. I would like to have the body and header fonts a bit smaller (consistently)…but only when watching on a desktop fullscreen. On smaller screens I want these to be the same as default.

I changed:

html {
font-size: 54.5%;
}

But now everything gets smaller and on smaller screens the font gets too small! Which seems logical.

But I am missing some fundamentals here. 54.5% of what? What is 100%

I have searched for articles on font use in twenty fifteen but didn’t find anything.

Hope someone can shed some light on these font sizes.