To change the nm-applet icon to a dark theme in polybar, you can follow these steps:

1. Open your polybar configuration file. Usually, it is located at ~/.config/polybar/config.

2. Find the module section for the nm-applet in the configuration file. It should look something like this:

   [module/nm-applet]
   type = internal/network
   interface = wlan0
   format-connected = <label-connected>
   format-disconnected = <label-disconnected>
   label-connected = %essid% %ip%
   label-disconnected = Disconnected
   

3. Add the label-connected-foreground and label-disconnected-foreground options to the module section. Set the values to the desired color for the connected and disconnected states. For example:

   [module/nm-applet]
   type = internal/network
   interface = wlan0
   format-connected = <label-connected>
   format-disconnected = <label-disconnected>
   label-connected = %essid% %ip%
   label-disconnected = Disconnected
   label-connected-foreground = #FFFFFF
   label-disconnected-foreground = #AAAAAA
   

   Replace #FFFFFF and #AAAAAA with the appropriate color codes for your dark theme.

4. Save the configuration file and restart polybar for the changes to take effect.

Now, the nm-applet icon in polybar should be displayed with the dark theme colors you specified.