My Question is about all the sims in the game.
I am not sure if it is the right place to post. If it's not the right place to post it please just move the thread to the right place.

I want to change all the sims' skins at once. Is there any way to do this?!

For example:
I want to replace all default Light skins with one custom skin for all Light skin tone sims,
and all default Normal skins with one custom skin for all Normal skin tone sims,
and all default Medium skins with one custom skin for all Medium skin tone sims,
and all default Dark skins with one custom skin for all Dark skin tone sims...
Instead of changing skin tones, sim by sim, which will take much time...

I have the 4 custom skins I want to use for every skin tone, and I have SimPE but I am not sure how to do this and if I CAN do this.

I hope to find an answer as soon as possible... I will try to do something about this myself, but I am waiting for an answer in case this didn't go well...

Thank you.

It is definitely possible.
I never figured out how to make a default replacement myself, but a clumsy first method I used ages back was downloading default replacements for all four and replacing the textures with those of the skins I wanted for each in SimPe.
Then I used Phaeno's correlated skintones for more variety, she wrote a tutorial for those found here: http://www.modthesims.info/showthread.php?t=481869 (you'll need her template, it's linked there).
And because I'm a bit bonkers and 16 skintones still weren't enough, I used this tutorial: http://rikkulidea.livejournal.com/21779.html (I think it's chapter six) to change values and townify lots of custom skintones. However, those will not change your existing Sims, you'll need a default replacement for that. But it's useful when adding even more... been a while since I finished that project, but I have somewhere around 70 skintones in my game.

EDIT: Looky here, I thought it was odd that I didn't know how to do it myself, there are several tutorials here!
http://www.modthesims.info/showthread.php?t=194390 (with pictures and templates)
http://www.modthesims.info/showthread.php?t=129993 (a simplified method)
http://www.modthesims.info/showthread.php?t=222953 (mixed methods)

The easiest way is to download a set of skin default replacements you like. Some creators post both custom and default versions of their skins, and occasionally someone else will have made defaults. You can find a lot by googling. If they don't exist as defaults, the tutorial links above look like they'll work fine.

If you want to use regular custom skins (non-defaults), you have to change the skin for each individual sim in SimPE. It's a lot more work.