discbox: Fix disc label color
A previous commit fixed the style of the checkboxes in AlbumWidget by removing the 'content-view' css style of the AlbumWidget. This indeed fixed this issue but introduced a new one: the disc labels now have a wrong color.
This new issue is fixed by updating the disc-label css style. The DiscBox activatable property is also set to False to prevent a color change on DiscBox hover.
Related: 4667eee3