Look & Feel > Manage Skin Sets & Templates
And edit your themes css, it is in "ipb_styles.css".
This is how it looks like in my ipb_styles.css:
a:hover { color: #1e1e1e; }
::-moz-selection { color: #fff; background: #6baa1b; }
::selection { color: #fff; background: #6baa1b; }
You need to change this last line, background color (background: #6baa1b). That's how it looks like in theme that I'm using.