Rubberized Shells (#20270)

* Added a 'Rubber' skin preset for Shells in character creation, which
makes their skin look cheap and uncanny.
* Added backend support for species to have different max and min skin
tone values, currently unused.


![image-2](https://github.com/user-attachments/assets/488efd17-b551-4aec-815d-ef62041b9086)

![image-3](https://github.com/user-attachments/assets/4d650019-cf14-4860-bf38-5af1f8f4dae6)

![image-2](https://github.com/user-attachments/assets/2d7d400f-47d5-45ee-a45b-47199d36fe74)

code/modules/mob/living/carbon/human/species/species.dm

@@ -326,10 +326,18 @@
 	)
 
 	var/zombie_type	//What zombie species they become
-	var/list/character_color_presets
 	var/bodyfall_sound = /singleton/sound_category/bodyfall_sound //default, can be used for species specific falling sounds
 	var/footsound = /singleton/sound_category/blank_footsteps //same as above but for footsteps without shoes
 
+	/// Sets the base "tint" of the species' sprite, which is then adjusted by the skin tone
+	var/list/character_color_presets
+
+	/// The lower bound for the skin tone value, the lower, the "lighter" they'll appear
+	var/lower_skin_tone_bound = 30
+
+	/// The upper bound for the skin tone value, the higher, the "darker" they'll appear
+	var/upper_skin_tone_bound = 220
+
 	var/list/alterable_internal_organs = list(BP_HEART, BP_EYES, BP_LUNGS, BP_LIVER, BP_BRAIN, BP_KIDNEYS, BP_STOMACH, BP_APPENDIX) //what internal organs can be changed in character setup
 	var/list/possible_external_organs_modifications = list("Normal","Amputated","Prosthesis")
 	/// These are the prefixes of the icon states in talk.dmi.

code/modules/mob/living/carbon/human/species/station/ipc/ipc_subspecies.dm

@@ -66,7 +66,8 @@
 		)
 
 	base_color = "#25032"
-	character_color_presets = list("Dark" = "#000000", "Warm" = "#250302", "Cold" = "#1e1e29")
+
+	character_color_presets = list("Dark" = "#000000", "Warm" = "#250302", "Cold" = "#1e1e29", "Rubber" = "#000f36")
 
 	sprint_temperature_factor = 1.3
 	move_charge_factor = 0.85