@sharonyue You would need to modify the files inside your theme folder, e.g. /node_modules/nodebb-theme-persona/. Keep in mind, these files are overwritten when you update.
Specify Custom Theme at install time
Is there a way to override which theme should be running by default when starting NodeBB for the first time?
You should be able to pass in the defaultTheme during setup now.
Thank you so much!