Save a copy of 'operaInstallDir/Styles/im.css' somewhere safe first!.
Right click on style titles and choose 'Save target as ...' - 'operaInstallDir/Styles/im.css'
(*nix users will need to be logged in as root). (I also have instructions on how to find and edit this file on Mac.)
Some styles (XP & babe) offer more than one way to use them - see inside the files for details.
Thx to
Enhanced, Olli, Junyor, NSaibot, *Someone*, Toman, achhoo, *forgotten*, Juan Sanches Villalobos Ramirez and anyone else who
may have contributed in any way, no matter how small, like saying yes/no/ugly/pretty and even if their so called help was just mindless abuse ...
See also my tips for how to disable chat features using CSS. See also Toman's Opera IRC styles.