You may have noticed that there is a difference in the final image when rendering the same scene in Picture Viewer and Thea Darkroom. The reason for that is that the image in Picture Viewer is being Gamma Corrected twice.
To make Picture Viewer behave correctly, do the following:
Go to Project Settings (From the menu: Edit > Project Settings or from the attribute manager: Mode > Project). In the Project Settings tab, uncheck the Linear Workflow option and everything will be back to normal.