Remove default theme's code from chrome/mozapps

```
$ git diff --name-status master a87d388 -- theme/chrome/mozapps/
M       theme/chrome/mozapps/downloads/downloads.css
D       theme/chrome/mozapps/extensions/addon-border.png
D       theme/chrome/mozapps/extensions/addons-category-border-rtl.png
D       theme/chrome/mozapps/extensions/addons-category-border.png
M       theme/chrome/mozapps/extensions/extensions.css
M       theme/chrome/mozapps/extensions/extensions.svg
D       theme/chrome/mozapps/extensions/off.png
D       theme/chrome/mozapps/extensions/on.png
M       theme/chrome/mozapps/extensions/rating-not-won.png
M       theme/chrome/mozapps/extensions/rating-won.png
```

I think, copy-paste all code from the default theme is overhead in this case. Can we use "style" (in chrome.manifest) for customization interesting pages (downloads window, about:addons)?

P.s. a87d388 is commit in `vanilla` branch.