Make dark mode black instead of the system window grey

The panel is a full-height strip that sits beside your work, where the
system grey reads as a half-lit window next to a dark editor. Dark mode
is now pure black, with card and row colours in one Theme file so both
the panel background and the strip under the titlebar match.

Sources/Prompt/PanelController.swift

@@ -56,6 +56,10 @@ final class PanelController: NSObject {
         panel.isMovableByWindowBackground = true
         panel.hidesOnDeactivate = false
         panel.isReleasedWhenClosed = false
+        panel.isOpaque = true
+        // Match the SwiftUI content so the titlebar strip and any resize flash
+        // do not show the system grey through.
+        panel.backgroundColor = Theme.background
         panel.collectionBehavior = [.canJoinAllSpaces, .fullScreenAuxiliary]
         panel.contentView = NSHostingView(rootView: PanelView(store: Store.shared))
         panel.setFrameAutosaveName("PromptPanel")

Sources/Prompt/PanelView.swift

@@ -40,6 +40,7 @@ struct PanelView: View {
             composerField
         }
         .frame(minWidth: 300)
+        .background(Theme.backgroundSwiftUI)
         .background(shortcutButtons)
         .overlay(sectionSwitcher)
         .overlay(syncOverlay)
@@ -248,7 +249,7 @@ struct PanelView: View {
 
     private func rowBackground(_ item: PromptItem) -> Color {
         if selection.contains(item.id) { return Color.accentColor.opacity(0.25) }
-        return Color.primary.opacity(0.05)
+        return Theme.rowSwiftUI
     }
 
     @ViewBuilder

Sources/Prompt/Theme.swift

@@ -0,0 +1,28 @@
+import AppKit
+import SwiftUI
+
+/// Appearance-aware colours for the panel.
+///
+/// Dark mode is pure black rather than the system window grey: the panel is a
+/// full-height strip that sits beside your work, and the grey reads as a
+/// half-lit window next to a dark editor or terminal.
+enum Theme {
+    static func isDark(_ appearance: NSAppearance) -> Bool {
+        appearance.bestMatch(from: [.aqua, .darkAqua]) == .darkAqua
+    }
+
+    /// The panel's own background (also handed to the NSPanel so the area under
+    /// the transparent titlebar matches).
+    static let background = NSColor(name: nil) { appearance in
+        isDark(appearance) ? .black : .windowBackgroundColor
+    }
+
+    /// Card background, kept subtle but visible against pure black.
+    static let row = NSColor(name: nil) { appearance in
+        isDark(appearance) ? NSColor(white: 1, alpha: 0.07) : NSColor(white: 0, alpha: 0.05)
+    }
+
+    /// Same colours for SwiftUI.
+    static let backgroundSwiftUI = Color(nsColor: background)
+    static let rowSwiftUI = Color(nsColor: row)
+}