An instance that can be somebody else's

A name, a tagline, a logo, a favicon and the launcher icons derived from it; the
Middle-earth strings as data; themes as token sets; and a stylesheet for what
none of that reaches. All four are on one page, in one settings group.

The snapshot is a Jinja global over a process-level cache, because render() has
no session and four render paths never reach it at all -- the sign-in page, the
error pages, the offline page and the SSE fragments. A context value would have
had to be threaded through every one and would still have missed those. It being
a global is also what lets mark() branch on an uploaded logo without any of its
six call sites learning about branding; the macro that renders the sidebar link
is called brandlink now, because a macro imported as `brand` shadows the global
for the whole template and took out every page at once.

Defaults in code and overrides in the database, as the prompt fragments do, with
one difference stated in the module: an empty fragment means off, an empty
flavour string means the shipped wording. And blanked rather than dropped --
settings_store.update merges, so an omitted key leaves what was stored last time
and "I typed the default back in" would store something different from "I changed
nothing".

A custom theme sets a handful of tokens and inherits the rest, and the
inheritance is a CSS fact: tokens.css matches [data-base="shire"] as well as
[data-theme="shire"], so a custom light theme lands on parchment rather than four
light colours on near-black. Values are validated on read rather than on save,
because a theme written straight into the settings table still has to produce a
stylesheet that parses -- a `}` in a value ends the rule and silently breaks
every rule after it. The soft variants are derived from the accent, or a changed
accent leaves focus rings in the old hue and reads as half-working.

/branding.css is a route, not an inline block: an external stylesheet has no HTML
context to escape from. The link carries a content hash, so a save is not left to
the browser's cache, and it is deliberately outside the service worker's precache
list, which is versioned by the release.

The instance name moved off /admin/general rather than being duplicated there.
An upgrade keeps it: the general row is read as a seed exactly while the branding
row has never mentioned the name, which is `key in row` and not `row[key] is
truthy` -- the two read alike would resurrect the old name underneath a cleared
one.

The theme list stops being a hard-coded pair in five places. Every failure mode
in that area is silent, so it is driven under a DOM stub as well as tested.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/api/branding.py

@@ -0,0 +1,65 @@
+"""Serving what an administrator customised.
+
+Both routes here are deliberately **unauthenticated**, and for the same reason
+the manifest and the offline page are: the sign-in page needs the logo before
+anybody has signed in, and a browser fetches a stylesheet and a launcher icon
+outside any page's session.
+
+What that exposes is a file an administrator uploaded on purpose to be shown to
+everybody, under a random filename, in a format that cannot execute in an
+`<img>` — `services/uploads.py:ALLOWED_TYPES` is what makes the last part true,
+and it is why SVG is not in it.
+"""
+
+from __future__ import annotations
+
+from fastapi import APIRouter, HTTPException, Response, status
+from fastapi.responses import FileResponse
+
+from lembas.services import branding as branding_service
+from lembas.services import uploads
+
+router = APIRouter(tags=["branding"])
+
+
+@router.get("/branding.css", include_in_schema=False)
+async def branding_css() -> Response:
+    """The custom themes and the custom CSS.
+
+    A route rather than an inline `<style>` in `base.html`, which is a security
+    property before it is a caching one: an external stylesheet has no HTML
+    context to escape from, so an administrator's CSS cannot become markup
+    however it is written. Inline, the same text would be one `</style>` away
+    from being a script on every page.
+
+    Cached hard and busted by a query string. `base.html` links this with
+    `?v={{ brand.revision }}`, a hash of everything below, so the URL changes
+    exactly when the stylesheet does. Without that the browser's cache is what
+    decides when a rebrand takes effect, which is a save that looks like it
+    worked and did nothing.
+    """
+    brand = branding_service.snapshot()
+    return Response(
+        branding_service.stylesheet(brand),
+        media_type="text/css",
+        headers={"Cache-Control": "public, max-age=604800"},
+    )
+
+
+@router.get("/branding/{filename}", include_in_schema=False)
+async def branding_asset(filename: str) -> Response:
+    """A logo, a favicon, or a launcher icon derived from one."""
+    path = uploads.branding_image_path(filename)
+    if path is None:
+        raise HTTPException(status.HTTP_404_NOT_FOUND, "No such file.")
+    return FileResponse(
+        path,
+        media_type=uploads.media_type_for(filename),
+        # Public, unlike a model avatar: this is served to somebody who is not
+        # signed in, so there is nothing private to keep out of a shared cache.
+        # Names are random, so a replacement is a new URL.
+        headers={
+            "Cache-Control": "public, max-age=604800",
+            "X-Content-Type-Options": "nosniff",
+        },
+    )