PDF "open in external window" gets no theme: `resolve-styles!` links only `style.css`

[CR0CKER]

What happens

Opening a PDF and clicking Open in external window gives a window that is always stock
Logseq: no theme, and no custom.css. The same PDF viewer in the main window is themed
normally.

This affects every theme, not one in particular — nothing a theme or a graph's
custom.css can do reaches that window.

Why

The external window is not a route. It is built by hand in
src/main/frontend/extensions/pdf/windows.cljs
(setup-win!), and its stylesheets come from resolve-styles!:

(defn resolve-styles!
  [^js doc]
  (doseq [r ["./css/style.css"]]          ; <- the app's own sheet, and nothing else
    (let [^js link (js/document.createElement "link")]
      (set! (.-rel link) "stylesheet")
      (set! (.-href link) r)
      (.appendChild (.-head doc) link))))

Registered themes and custom.css are injected into the main document, which this
window is not part of, so they never appear in it.

Everything else about the window is already set up for theming — setup-win! copies the
theme mode onto the new document and marks it:

(set! (.-dataset doc-el) -theme (str theme-mode))   ; data-theme is set
(resolve-classes! doc)                              ; adds .is-system-window

So a theme's own selectors (html[data-theme] …) would match immediately. The stylesheet
is simply absent.

Suggested fix

Have resolve-styles! also carry whatever the main document is using:

• the active theme's stylesheet (the registered theme's URL, as the main document links
  it), and
• the graph's custom.css.

A plugin can work around this today by patching window.open on the host renderer and
appending the sheets to the new document — but that is host monkey-patching from what is
often just a theme, and it is the kind of unofficial host access the plugin SDK warns is
unsupported on the Marketplace. It also has an ordering trap worth noting for the fix
itself: setup-win! appends style.css synchronously after open() returns, so a sheet
added any earlier loses every specificity tie against it. Whatever links the theme should
do so after style.css, exactly as the main document does.

Steps to reproduce

1. Enable any theme (a marketplace theme, or a custom.css with something obvious like
   body { background: red }).
2. Open a PDF asset in the PDF viewer — it is themed.
3. Click Open in external window.
4. The new window is unthemed.

Environment

• Logseq OG, version/file @ 6e7afa8eb (code verified against that tip)
• Linux / Wayland (GNOME), but nothing here looks platform-specific
• Reported from writing a GNOME/Adwaita theme, where the external PDF window is the only
  surface the theme cannot reach