[Mijae]

Not sure if this should be here, if not the mods should feel free to move it wherever it should be

What this does: its a BB code that will make all text between the [spoiler][/spoiler] hidden until you hover your mouse over it.

Step #1:

Go to Styles & Templates >> your style >> All Style Options, click Go, scroll down until Additional CSS Definitions, and there add:

.spoiler
{
color: #000000;
text-decoration: none;
background-color: #000000;
}

Step #2:
Now go to Custom BB Codes and select Add New BB Code. There, add the following information:

Title: Spoiler
Tag: spoiler
Replacement: <span class="spoiler" onmouseover="this.style.color='#FFFFFF';" onmouseout="this.style.color=this.style.backgroundColor='#000000'">{param}</span>
Example: [spoiler]Stuff goes here[/spoiler]
Description: The spoiler tag will hide the text between the tags, and will appear only when you put your mouse over the hidden text.
Use {option}: set to No

And that's it! Now you have a fully functional spoiler code!

Demo: Check out this thread

Additional info: this codes is valid XHTML 1.0 Transitional.

Enjoy