Make the text content follow Material You scheme

[Skythrew]

For the moment, the text content of any chapter (in PartScreen) is forced to be black over a white background to improve readability. It would be nice to make this text follow the Material You color scheme too.

Thus, I think we should not force neither the color of the next nor the background color. In a next release, when a preferences screen will be implemented, we'll let the user choose if he wants this UI to follow the Material theme or not.

Initial issue: #8