I cannot find the option for dark mode on my iPhone, thoughts & thank you
DTTG adopts the system-wide settings, which are to be found in iOS Settings/Display & Brightness. In addition, DTTG has a setting (reached via the slider icon in the bottom right of the screen when displaying the global inbox) called “PDF auto dark mode”.
If your iPhone is in dark mode, so too will DTTG be. The background of PDFs will be darkened when iOS/DTTG goes into dark mode if you have additionally activated “PDF auto dark mode”.