Design done and approved in PLANET-3574
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Similar to what we did with spacing, we should define specific proportional values for font sizes and stick to them, instead of using arbitrary values. This will increase consistency and maintainability.

This is a design task, in order to see design implications first. This is the suggested set, based on what we already use:

font-base-size: 16px

font-size-xxs: 0.875rem;
font-size-xs: 0.9375rem;
font-size-sm: 1rem;
font-size-md: 1.25rem;
font-size-lg: 1.375rem;
font-size-xl: 1.75rem;
font-size-xxl: 2.5rem;
font-size-xxxl: 3rem;