Background color when 'external editor' option is enabled.

This theme is just awesome. Thanks for your work.
But the background color when 'use external editor' option is enabled, seems to be from the original Arduino theme which does not match the whole theme.

I had to change the color code on the line below to on theme.txt

```
# color to be used for background when 'external editor' enabled
editor.external.bgcolor = #c8d2dc
```