Using Brighten Live View Setting
There is an option under Camera settings to adjust the shutter speed for LiveView and photos to help with dark LiveView preview.
Press ctrl-3 to display the Camera settings dialog, and enable Brighten Live View. You can then set the shutter speed for Live View and Photos separately. This feature requires the camera be set to M or Shutter Speed (Tv) mode, since it will change the shutter speed when it takes the photo and when it displays live view. For Manual (M) mode, you will need to adjust the ISO and aperture accordingly to the shutter speed values you have setup.