Readium colGap vs pageGutter — pick the right CSS var or break layout

Two Readium-CSS vars sound interchangeable but mean opposite things: --RS__colGap controls the gap BETWEEN columns within a single page (only effective when column-count > 1 on that page), while --RS__pageGutter controls the gap between pages in spread/paginated view. Both default to 0. If you add a UI slider that overrides --RS__colGap to a non-zero default thinking itll widen the visible gap between the two pages of a spread, you instead force Readium to render an extra inner column on every page — so 'Columns: 1' displays 2 cols, 'Columns: 2' displays 3 cols with overflow bleeding off the edges. Diagnostic: if user reports a paginated reader showing N+1 visible columns when N is set, suspect a non-zero colGap override. The fix is either set colGap default back to 0 (matching Readiums own default in css/dist/ReadiumCSS-default.css) or switch the slider to target --RS__pageGutter instead. Companion rule: when adding any pref that overrides an existing CSS var, default it to the vars current default to avoid silent regressions for existing users on first upgrade.