Font-Size in Theme

For example the property-grid from component-examples.
If it is compiled without any style the font-size is 13px.
If it is compiled with the theme “kenney” for example it changes to 16px.
I added:

body {
   font-size: 13px;
}

in css but it is ignored.
If the font-size is set in .html it works normal.

Cheers,
Domi