Add support for dired-subtree

Added faces for dired-subtree. I originally struggled with whether or not to use different dark background colors to represent the depths, but then I referred to [comment](https://github.com/protesilaos/modus-themes/blob/613f95341246746c3def202e67b27a315560d6f4/modus-themes.el#L2180-L2183) in modus-theme and found that it worked well, as there're only four different dark background colors provided in spacemacs-theme, but dired-subtree defines six subtree depth faces.