Use 3 param method for converting r,g,b into css string.

This avoids creation of temporary arrays to pass them into the util
method. Also using "arguments" is more expensive then passing in 3
parameters.

FYI, here the benchmark which shows the cost of using 'arguments':
http://jsperf.com/arguments-or-explicit-use
Can you add a constant here, e.g. `var BACKGROUND_ENLIGHT = 0.7` (placed after the comment above) to not use magical 0.7 three times?

> the rounded result could be different however.

I don't really like having round in color operations. Also 255 multiplication can be moved just before makeCssRgb, e.g.:

```
var r = BACKGROUND_ENLIGHT * (1.0 - color[0]) + color[0];
var g = BACKGROUND_ENLIGHT * (1.0 - color[1]) + color[1];
var b = BACKGROUND_ENLIGHT * (1.0 - color[2]) + color[2];
content.style.backgroundColor = Util.makeCssRgb((r * 255) | 0, (g * 255) | 0, (b * 255) | 0);
```