check if theme_unfocused_selected_bg_color exist

GTK's default behavior is to use `selected_color` as a fallback if `unfocused_selected_color` does not exist.

Many themes rely on this behavior and do not set this color (almost all in Mint, for example).

With `misc-highlight-enabled` disabled, the theme works normally, but if it is enabled, the filename disappears on the white background.

This patch adds the fallback in Thunar's custom renderer.