Proposal for issue #52

At the moment, document-viewer doesn't specify any theme to use. That's in principle a very elegant solution. But the icon set for the action bar in document-viewer is so light that you can barely see the icons, when the system wide theme uses a light background color for the action bar.

Furthermore, the text color for the browser items is forced to be white in document-viewer/src/main/res/values/service_styles.xml and that will also cause problems with light system-wide themes.

In this pull request I propose to force the background color to be black with white text everywhere. Alternatively, you could use a differenct icon theme and/or stop forcing the textColor for the browser items to be black in service_styles.xml.