Privacy Browser 3.5.1 contains two important bug fixes relating to the the new Day/Night theme and how Privacy Browser now handles being restarted by the OS.
The first fixes a problem with the intents (links from other apps) not opening if Privacy Browser was restarted in the background by the OS. Privacy Browser would restore the previously open tabs but not load the new link.
The second fixes a problem with Privacy Browser not initializing correctly if the theme app theme was set to be the opposite of the OS theme. This would manifest differently on different devices, but would generally prevent bookmarks or even some links from loading on the first tab.