Component toolbar is partially white when it contains only one component

When the component toolbar contains not enough components to fill the whole width of the window, part of the toolbar is white instead of the default grayish background.

Steps to reproduce:
1. start the archetype
2. login to the CMS and open the channel manager
3. open the single channel
4. click 'Edit template'
5. click 'Add components'

Expected: the whole components toolbar has the same greyish color
Actual: part of the component toolbar is white instead of grey