docs(docs): document Bibata cursor package and GTK configuration

docs/desktop/umbriel.mdx

@@ -35,7 +35,7 @@ Umbriel watches these files and applies a valid save immediately; `umbriel confi
 | Keyboard | `us`, repeat delay 400 ms, rate 50, NumLock on | niri |
 | Touchpad | natural scroll, tap, left-right-middle, sensitivity 1.0 | niri |
 | Tablet, touch | mapped to `eDP-1` | niri |
-| Cursor | `capitaine-cursors-light` 24, hidden while typing, follows keyboard focus (so scratchpads open on the output focus moved to) | niri, Umbriel |
+| Cursor | `Bibata_Ghost` (dark translucent Bibata, AUR `bibata-cursor-translucent`) 28, hidden while typing, follows keyboard focus (so scratchpads open on the output focus moved to). Umbriel exports it as `XCURSOR_THEME` and over XSETTINGS; GTK apps on Wayland read the GNOME settings instead, set once with `gsettings set org.gnome.desktop.interface cursor-theme Bibata_Ghost` and `… cursor-size 28` (dconf, not managed) | Umbriel |
 | Layout | gap 8, presets 0.333 / 0.5 / 0.667, new columns 1/3 on every output, focus never centers, a lone column centers | niri |
 | Appearance | corner radius 10, no shadows, wallpaper-only blur on every window, tab bar hidden for one tab | niri |
 | Activation | apps may focus and reveal their windows (`focus_on_activate`), e.g. Noctalia notification actions | niri `honor-xdg-activation-with-invalid-serial` |