Add light/dark theme toggle; show the order card only after a result (!3) · Merge requests · Hop179 / Colocation Calculator

Aiho Tehisaru requested to merge feature/theme-toggle into trunk Sep 25, 2026

What

Theme toggle in the header (moon/sun icon). Defaults to the OS preference; the first click stores an explicit choice in localStorage (kolo-theme). A data-theme attribute, applied by a tiny head script before first paint, overrides prefers-color-scheme in both directions, so there is no flash of the wrong theme. color-scheme is set per theme so native controls follow. Blocked storage degrades to a non-persistent toggle.
Order Colocation card is now rendered hidden and shown only while the calculated price (#output) is visible. It hides again on empty input or errors. No KOLO_ORDER_URL still means no card at all.
No external resources. Audited the page: system font stack, inline CSS/JS and inline SVG only, nothing to vendor. Added a test so a CDN script, webfont or stylesheet can't sneak in later.

TDD: new tests in internal/api/api_test.go (toggle markup, CSS selectors, head script before <style>, hidden order card, no external refs), seen failing first. go test ./... passes.
Real browser check (headless Chromium, 23 assertions): OS light/dark, toggle both ways, persistence across reload, no flash, blocked localStorage, order card visible only with a result (hidden on clear and on validation error), zero external requests, no console errors. Visually checked both themes at phone width.

No API, config or Go runtime changes.

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