[junaid]

it is relatively easy with new version.
{if strstr($current_category.categoryid_path, 'xyz')}{include file=xyz.tpl}{/if}

xyz.tpl can be your header, footer right nav, subcat.tpl or even home.tpl or even .css file.

it would be a headache to maintain all the files in longer run therefore i would recommend having css and top portion of site changed.

i hope it helps.