Dunno how Opera's custom CSS works, but you'd just find out how to load a custom stylesheet and put this code in it:
Not sure if it lets you limit the CSS to certain domains or what-have-you.
Edit: It tells you how to do site-specific user CSS here:
http://operawiki.info/OperaUserCSS
Code:
/*
** OcUK colour scheme changer
** By Rob Miller
*/
* { color:#000 !important; }
textarea, input { background:#ddd !important; }
body, .page, .thead, .tfoot, .alt2, .alt2Active, .panel { background-color: #fff !important; }
.alt1, .alt1Active, .panelsurround, .vbmenu_option { background-color: #fff !important; }
.alt2 { background:#dfdfdf !important; }
.vbmenu_control, .vbmenu_control *, .vbmenu_hilite, .vbmenu_hilite *, .tcat, .tcat * { background-color: #006598 !important; color:#fff !important; }
.vbmenu_popup { border: 1px solid #ccc !important; }
Not sure if it lets you limit the CSS to certain domains or what-have-you.
Edit: It tells you how to do site-specific user CSS here:
http://operawiki.info/OperaUserCSS