[R-Forge #2357] chart_theme background color is ignored

Submitted by: Garrett See
Assigned to: Nobody
[R-Forge link](http://r-forge.r-project.org/tracker/index.php?func=detail&aid=2357&group_id=125&atid=544)

If we define

```
thm <- chart_theme()
```

thm$col$bg does not seem to do anything

```
getSymbols('SPY', src='yahoo', from='2012-01-01', to='2012-11-01')
thm$col$bg <- 'black'
chart_Series(SPY, theme=thm) # shouldn't this have a black background?
```

Start by reproducing the example using chart_theme(), getSymbols(), and chart_Series() with thm$col$bg set to 'black'. Trace how chart_Series() applies the supplied theme and verify whether the background color is ignored. Done means the example renders with a black background.

Written by the indexing model from the issue text.