Welcome to the forum, donpedro.
There is no hex code for "transparent" -- every hex code actually is a color. Each pair of "hex code" characters defines an R-G-or-B value.
If you want a transparent color in a CSS rule, you use the word "transparent" -- but that means "don't color the background of this element independently, use the background color of its containing element".
Browsers, by default, use the system background system color when no background color is explicitly declared for the entire body, or when there's an empty window. And by default on most systems, this is a light gray. But both the system default background color and the browser's default background color can be reset by the user.
So you can't depend on the background color being light gray, or any color at all. In fact, I keep my browser default background color set to a light blue. So if I forget to declare a background color in my code, I see that immediately when I test the page.
IE has some proprietary coding that lets you tap into the system color settings, but I've never played around with it. You see the css for it when you save a Word document as HTML (if you ever do such a horrible thing!)