ForgeTrust.AppSurface.Theming

ForgeTrust.AppSurface.Theming defines immutable semantic light/dark pairs for AppSurface-owned UI. It is deliberately not a site-wide design system: it does not style application components, choose a user preference, persist a cookie, inspect a request, implement tenant policy, or load remote theme packs.

For an ASP.NET Core application, begin with the Web theme-pairs quickstart. ForgeTrust.AppSurface.Web references this package and supplies the Razor integration. Install this package directly only when authoring another package-owned adapter.

Release Guidance

AppSurface ships as a coordinated package family. Before installing this package from a prerelease feed, check the package chooser and release hub for current release risk, migration guidance, and readiness.

Contract

A pair has one canonical lowercase identifier (letter-led, 63 characters or fewer, with only lowercase letters, digits, and single interior hyphens) and a complete Light and Dark set of these roles:

Shared role values must be opaque #RRGGBB colors. Configuration seals them at registration time and checks text-role contrast at 4.5:1 plus non-text affordances at 3:1 against every shared surface. The built-in AppSurfaceThemePair.AppSurface() pair is the default example.

using ForgeTrust.AppSurface.Theming;

services.AddAppSurfaceTheming(options =>
{
    options.DefaultTheme = new AppSurfaceThemeId("appsurface");
    options.DefaultMode = AppSurfaceThemeMode.System;
    options.Pairs.Add(AppSurfaceThemePair.AppSurface());
});

System emits both branches and lets browser CSS select prefers-color-scheme. Light and Dark emit only the selected branch. This is host configuration, not per-user policy.

For a presentation-only browser choice, use the Web-layer browser-local preference adapter. It is an explicit opt-in that preserves one canonical HTML document, stores only light or dark in browser-local storage, and falls back to this package's System CSS. Account synchronization, tenant selection, request-aware content, consent decisions, and content-varying themes remain application policy and do not belong in this neutral package.

Package-owned adapters that need a fail-closed boundary can call AppSurfaceThemeRegistry.IsSafeResolution(resolution). It applies the same identity, role, and contrast contract as startup registration without depending on or allocating a Web document.

Application-specific settings

An application can provide typed settings to a package-owned adapter through IAppSurfaceThemeExtensionProvider<TSettings>. The neutral package treats the setting as opaque: it neither validates, serializes, logs, nor renders it.

sealed record AcmeThemeSettings(string ProductMarkUrl, bool ShowReleaseRail);

sealed class AcmeThemeSettingsProvider : IAppSurfaceThemeExtensionProvider<AcmeThemeSettings>
{
    public bool TryGet(AppSurfaceThemeId themeId, out AcmeThemeSettings settings)
    {
        if (themeId.Value == "appsurface")
        {
            settings = new AcmeThemeSettings("/assets/acme-mark.svg", true);
            return true;
        }

        settings = null!;
        return false;
    }
}

Register the provider and call AddRequiredThemeExtension<TSettings>() when every configured pair must have settings. The neutral registry checks provider presence and calls TryGet once per sealed pair when it is first resolved; it reports ASTHEME201 for a missing provider and ASTHEME202 for a missing or null pair setting.

services.AddSingleton<IAppSurfaceThemeExtensionProvider<AcmeThemeSettings>, AcmeThemeSettingsProvider>();
services.AddRequiredThemeExtension<AcmeThemeSettings>();

Use this seam when a specific adapter needs app-owned configuration. Do not add arbitrary metadata bags or promote application concepts into shared semantic roles. The application still owns settings-schema, asset, and authorization validation.

Diagnostics and pitfalls

Every startup diagnostic follows Problem / Cause / Fix / Docs ordering and is safe to surface to an operator:

• ASTHEME001 — no usable pairs or unsupported mode.
• ASTHEME002 — default pair is not registered.
• ASTHEME003 — missing/invalid pair identity.
• ASTHEME004 — duplicate canonical pair id.
• ASTHEME005 — a role is not one opaque #RRGGBB value.
• ASTHEME101 — a role fails its required contrast ratio.
• ASTHEME201 — an opted-in extension provider is not registered.
• ASTHEME202 — an opted-in extension provider has no setting for a registered pair.

The Web adapter preserves a conflicting host color-scheme declaration and exposes data-as-theme-color-scheme-conflict="true"; resolve the conflict explicitly rather than expecting AppSurface to overwrite host CSS.

Do not use this package for a switcher, local-storage/cookie precedence, tenant lookup, or client-hint selection. The supported browser-local adapter lives in AppSurface Web; cookie, account, tenant, client-hint, and content-varying approaches still require a separate privacy, CSP, caching, and first-paint design. Related follow-on policy work is tracked in #705, #706, #707, and #708.