Font sizing, UI spacing and colours are specified using css variables. See scss/0-settings/_config.scss if you use Sass, else visit the Presentation: Styles tab and select the stylesheet named default.
Variable font-scaling: the base font size adjusts smoothly between configurable limits.
Heading sizes use the Major Third type scale, and include proxies (like <h1 class="h3-style">) so font sizing is separated from semantics.
If the container has a class of b_typeset (like the excerpt and body fields in all-grid), extra typographic rules are applied.
