The lightbox toolbar background does not wrap completely the buttons with silk theme

Steps to reproduce:

1. Enable lightbox from Administration -> Look&Feel -> Lightbox
2. Switch to Silk Icon Theme from Administration -> Look&Feel  -> Themes -> Icon Themes
3. Go to a page and add a image inside the content
4. Hover the image

Expected result: the lightbox toolbar is correctly displayed

Actual result: the toolbar while background does not wrap well the buttons

Note that the buttons are still usable, even if the display seems broken