Two commits for scroll hints had impact on FileSystem dock style:

- f187b8b2bf introduced `TreeSecondary` for the tree pane, but only applied it in horizontal split mode.
- a45bdce763 removed the existing `ItemListSecondary` from the file list in vertical split mode.

The combination results in vertical split and tree-only not having background styles while horizontal split does.

Apply `TreeSecondary` and `ItemListSecondary` styles in all split modes. This gives consistency between the modes and with other elements.

Fixes #119359