Update How to force Pi to use light theme (*NIX systems)

How-to-force-Pi-to-use-light-theme-%28%2ANIX-systems%29.md

@@ -3,3 +3,8 @@ Simply run Pi with the `FYNE_THEME` environment variable to `light`. Like so:
 ```bash
 FYNE_THEME=light pi-im
 ```
+
+You can also do the same for a dark theme:
+```bash
+FYNE_THEME=dark pi-im
+```