Slider has one-off skins, like sam-dark and capsule that are being built into /build/slider-base/assets/skins/.

The following code does not load these skin files:

YUI({
skin: {
overrides: {
slider: [
'sam', // The default skin
'sam-dark', // Suited for dark backgrounds

'capsule', // You only need to include one skin
'capsule-dark', // in the overrides section unless you
// are using multiple skins on the same page
'round',
'round-dark',

'audio-light',
'audio'
]
}
}
}).use('slider', function ( Y ) {

Test pages:
yui3/src/slider/tests/manual/skins.html
http://stage.yuilibrary.com/yui/docs/slider/slider-skin.html