Feature request: wallpaper (background image) support in the new theme system #5661

The new theming system in the nightlies (.1025+) is excellent — I moved my
whole custom palette into it the day it appeared. This is a request for the
one visual layer it can't express yet: a background image behind the canvas.

What I built (works today, via CSS patching)

A desktop wallpaper that sits under the whole app, with the existing glass
surfaces doing all the work of keeping content readable. The composer is the
star: its translucent glass lets the image ghost through the prompt box, and
the native glass-opacity slider already gives users the control they need.

Mechanically it's small — which is why I think it would make a clean native
feature:

1. One image layer on the root (I use html::before, position: fixed,
   cover, with a theme-colored gradient over it to set the ghosting level —
   the equivalent of a wallpaper opacity setting).
2. Four stacked bg-background/80 layers currently bury it (the inner flex
   container, the scroll container, main, and body). They need to be
   transparent when a wallpaper is active — everything above them (cards,
   bubbles, composer, popovers, sidebar) keeps its own paint and stays
   readable with zero further changes.

That's the entire mechanism. No component changes needed — the app's existing
glass design language already composes beautifully with an image behind it.

Proposed shape

An optional field on the theme file (or a standalone appearance setting):

{
  "version": 1,
  "name": "Deep Veil",
  "appearance": "light",
  "colors": { /* ... */ },
  "wallpaper": {
    "image": "<data URI or file path>",
    "opacity": 0.15,        // how much image shows through the canvas color
    "fit": "cover",
    "position": "center"
  }
}

• opacity as "image strength" (canvas color over image) matches how people
  reason about wallpapers, and keeps every theme readable by default.
• The glass-opacity setting already covers the surface-translucency half.
• Import/export keeps working — a theme with its wallpaper travels as one
  file (data URI), which is a lovely property for sharing themes.

Happy to share the working CSS, screenshots, or test nightlies. Thanks for
the theme system — funny how being able to theme it the way I want made me use
t3code a lot more.