See OperaUserCSS for a tutorial
User Stylesheets are Cascading Style Sheets (CSS) where you define the defaults. You can use them to override page styles, fix deprecated tags like <wbr>, or to change a page formatting to your liking.
In Opera, User Stylesheets are invoked with the dropdown menu of the image toggle button on the addressbar, or from the View - Style menu.
Opera comes with 11 User Stylesheets, which can be used to debug pages, increase contrast, disable tables, and some other effects. If you edit some ini files, you can add more User Stylesheets to this list.
In addition to this, under Preferences - Page style - My style sheet, you can specify a permanent UserStyleSheet file where you can add your own styles.
In order to use a UserStyleSheet during normal browsing, go to Preferences » Advanced » Content » Style options... and toggle ‘My style sheet’ for Author mode.