dark-mode: add page (#5225)

pages/osx/dark-mode.md

@@ -0,0 +1,20 @@
+# dark-mode
+
+> Control macOS dark mode from the command-line.
+> More information: <https://github.com/sindresorhus/dark-mode>.
+
+- Toggle dark mode (turn it on if it's currently off, off if it's currently on):
+
+`dark-mode`
+
+- Turn dark mode on:
+
+`dark-mode on`
+
+- Turn dark mode off:
+
+`dark-mode off`
+
+- Check if dark mode is on:
+
+`dark-mode status`