Match window border color to app theme rather than OS theme (#958)

* MacOS: match window chrome to app light/dark mode when app setting is different from OS

* match window border to app theme on Windows

ui/controller/windowtheme_darwin.go

@@ -0,0 +1,16 @@
+//go:build darwin
+
+package controller
+
+/*
+void setWindowDarkMode(void* windowPtr, int useDarkMode);
+*/
+import "C"
+import "unsafe"
+
+// setWindowDarkTheme takes an unsafe.Pointer referencing the macOS NSWindow
+// and explicitly applies or clears the dark mode appearance.
+// Mode arg: 0 = auto, 1 = dark, 2 = light
+func setWindowDarkTheme(nsWindowPtr uintptr, mode int) {
+	C.setWindowDarkMode(unsafe.Pointer(nsWindowPtr), C.int(mode))
+}