The following page of the Disgaea RPG guide answers how to change the background in the Stronghold. We also tell you where to find the Stronghold Customization option.
Go to the Stronghold tab, click on the Menu icon in the upper right corner, and then select Stronghold Customization.
Select the available background theme and music and then click on Confirm. You can make these changes at any time.
You can get new Stronghold backgrounds as prizes during special in-game events. Making progress in the story also unlocks new backgrounds in the Dark Assembly.