Hello !
I jut wanted to change the color of the categories titles without changing the other links color of my forum !

did any one have the CSS code of this option please ?

I don't know about changing it in the CSS. Hopefully someone else will know.

but when you make your categories you can use the HTML code to change the color.

Code:

Example:
Category name: <font color=red>Category name goes here</font>

Just to make it more customizable than how Psichotic suggested

Code:

<font color="#FFFF80">Category</font>

Use the above code.
If you want to change colors. USE THIS Just replace the font #FFFF80 with the code on the website.

Hope this helps.