Use color substitution in default themes

So that it's usable with dark gtk themes as well.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>

themes/default-hdpi/themerc

-active_text_color=#0c0c0f
-active_text_shadow_color=#fcfcff
-inactive_text_color=#6c6c70
-inactive_text_shadow_color=#fcfcff
+active_text_color=active_text_color_2
+active_text_shadow_color=active_hilight_2
+inactive_text_color=inactive_text_color_2
+inactive_text_shadow_color=inactive_hilight_2
 button_offset=2
 button_spacing=0
 full_width_title=true

themes/default-xhdpi/themerc

-active_text_color=#0c0c0f
-active_text_shadow_color=#fcfcff
-inactive_text_color=#6c6c70
-inactive_text_shadow_color=#fcfcff
+active_text_color=active_text_color_2
+active_text_shadow_color=active_hilight_2
+inactive_text_color=inactive_text_color_2
+inactive_text_shadow_color=inactive_hilight_2
 button_offset=0
 button_spacing=0
 full_width_title=true

themes/default/themerc

-active_text_color=#0c0c0f
-active_text_shadow_color=#fcfcff
-inactive_text_color=#6c6c70
-inactive_text_shadow_color=#fcfcff
+active_text_color=active_text_color_2
+active_text_shadow_color=active_hilight_2
+inactive_text_color=inactive_text_color_2
+inactive_text_shadow_color=inactive_hilight_2
 button_offset=2
 button_spacing=0
 full_width_title=true