Bits and Pieces

There are two ways to do this;

Option 1: Custom CSS

/*
Name: Green on Black Selected Text
*/

::selection {
background: #000;
color: #7FFF00;
}

Option 2: Using Opera:Config

opera:config#Colors|SelectedBackground
opera:config#Colors|SelectedText

Option 1 allows you to make several variations and switch between them quickly, overwriting a sites preference. Option 2 is the least amount of work.