Theme tweaks

[mlim15]

• Lollipop does not support the theme variable used to make status icons dark on a light background. Status bar icons were thus invisible on lollipop. I have added a separate theme definition file for v21 and v22 that gives lollipop a grey status bar so that icons remain legible when using the light theme.
• The black theme now has its own blur gradient definition that leaves more of the screen black and fades more elegantly into the black appbar and nav buttons.
• Album art on the now playing screen now has rounded corners, a slight margin around the screen's edges, and a subtle drop shadow for a slightly more modern look.

My apologies for the barrage of pull requests today.