Logout button style to modify

Hello,

I would like to change css of the logout button to replace with a background other than the classical logout btn.
When I try to alter the code from the mod_sclogin, I don't know where the code needs to be modify.

The exact code to alter is the following one, where is it saved please.
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Many thanks
Michel

Job done - file to amend is the following one: facebook.php