[sailfish-browser] Remove hard dependency to Silica Theme

Would help readability if ```cssDefaultPixelRatio``` (and ```cssPixelRatioRoundingFactor```) would be prefixed with g.

```gCssDefaultPixelRatio``` and ```gCssPixelRatioRoundingFactor```. There are some other static variables that are not prefixed. If you'd like, you can push another PR later that would fix them as well :).