Fix appearance of images in Night mode.

- Explicitly apply a white background for images, so that images with
  transparency will appear correctly (since most images were intended to
  be viewed on a white background)
- However, for math formulae, keep the background transparent, but invert
  the image using a filter!

Change-Id: Ide2db30a6eeebb6a92a94740bfea0eb6719c9ce8