Exposing a framework CSS var as a user knob requires flow-level testing

Finding a framework CSS variable you can override (e.g. --RS__colGap in Readium) and wiring a slider to it feels like a clean 1-line patch, but the var being settable is not sufficient evidence that exposing it as a user knob is safe. Two patterns repeatedly bite: (1) the framework's internal layout math may not recompute related properties when your var changes — Readium computes column-width based on viewport but doesn't subtract column-gap, so colGap > 0 with column-count fixed at 2 causes viewport overflow and adjacent pages bleeding into view; (2) sibling features rendering in the same DOM area may rely on the var being at default — Readium's text-highlight renders backgrounds on per-word spans, so user-set word-spacing inserts uncolored gaps in the middle of a highlight. In both cases the visible-effect change (wider gap, wider word space) worked in isolation but the framework's OTHER systems didn't cooperate. Before exposing a third-party CSS-var override as a UI knob, test: zoom/font-size, every layout mode, highlights/selections, scroll vs paginate, theme switching.