This uses a modified form of Template:Hidden begin called "Hidden begin plus css" (User:Eitch/sandbox4). The modification, lifted from Template:Hidden, allows for the inclusion of css code in the hidden content.
Parameters:
- title = the title
- body = the content that's hidden
- titlebg = the color in the background of the title line (named or coded colors)
- bordercolor = the color of the border around the title line
e.g.
{{User:Eitch/sandbox5
|title=This is the title
|titlebg=#cccccc
|body=This will be shown when you click "show."
|bordercolor=red}}
Alternatively, you can specify two unnamed parameters. In this case, the first parameter is the title and the second parameter is the body.
e.g.
{{User:Eitch/sandbox5
|This is the title
|This will be shown when you click "show."
}}