Not a PT First user, but assuming they use the same methods(not always safe
):
1-Dark mode is a selection on the first tab of Preferences
2-One way is to enable the ALL group>Show volume automation(with the ALL group enabled, all tracks should go to this view)>drag any volume line down and all should move the same amount. Another option which I use a lot: I open the TRIM plugin, set it for -8 and then set the plugin to default to that setting. Then(after removing that instance), I hold the Alt key and add the TRIM plugin in the last slot of all tracks. Then immediately delete the plugin from any AUX tracks as you probably don't want to lower them(assuming they are fed by post-fade sends)