OS: Windows 10 - 64bit
When changing the background colour of thumbnails, the thumbnails of folders don't use this background colour which can seen at the very top (right) of the thumbnail.
Effect: Unexpected behaviour (minor)
To reproduce:
1. Start XnView browser
2. For "Thumbnail cell" in Tools » Settings | Browser » Thumbnail use a dark background colour.
3. Navigate a folder with subfolders and thumbnails in it. Select the subfolders and choose "View » Rebuild thumbnails"
4. For "Thumbnail cell" in Tools » Settings | Browser » Thumbnail use a bright background colour.
Actual behaviour (bug): Thumbnail cell has chosen (bright) background colour. The thumbnails of the subfolders still have the old (dark) background colour at the very top

Expected behaviour: Thumnbail cell has chosen (bright) background colour. The thumbnails of the subfolders also have the chosen (bright) background colour.