Well, depends. Do you plan on completely separating battle screen tone from map screen tone? If so, all you need to do is change this one line in the Game_Troop class:
to the following line:
And it should work as expected. Now, if you plan on having one or more battles somewhere in the game that require you to keep the same screen tone used in the map, the easiest way to set it up is by using a switch. Instead of the previous example, replace the line above with this one:
74 init_screen_tone if $game_switches[ (number) ]
Where (number) should be the actual ID of your control switch. So, if you choose to put in 20 as the switch ID, the battle screen tone will only be the same as the map's if switch 20 happens to be on.
Hope this helps!