A fork changes values, not files
Everything a project retunes lives in one stylesheet: the palette, the typefaces, the type scale, the spacing, the radii and the motion timings. The semantic layer above it is an audited set of relationships rather than a set of values, so changing a brand cannot quietly break the contrast it was checked against.
Typeface, weight and button radius are runtime options rather than compiled constants, which means a rebrand that touches only those needs no build at all.