Manual:$wgHandheldStyle

Details
Optionally, we can specify a stylesheet to use for media="handheld". This is recognized by some, but not all, handheld/mobile/PDA browsers. If left empty, compliant handheld browsers won't pick up the skin stylesheet, which is specified for 'screen' media.

Can be a complete URL, base-relative path, or $wgStylePath-relative path. To apply the Chick styles to the MonoBook HTML, try: $wgHandheldStyle='chick/main.css';

Will also be switched in when 'handheld=yes' is added to the URL, like the 'printable=yes' mode for print media.