Not that I know of – although you can change the look and feel of various pages on your site using a combination of conditional tags, WP functions and CSS.
what do you mean with two different css?
in which case?
you mean depeding on the time?
i.e. 8 a.m. = 8am.css and 8 p.m. = 8pm.css
if so, check this for different classes:
http://www.philworks.com/blog/2009/01/18/different-css-style-depending-on-the-time.html
or this here for different stylesheets:
http://css-tricks.com/forums/viewtopic.php?f=8&t=2207
you could also have different stylesheets randomly picked:
http://forum.mamboserver.com/showthread.php?t=61029
So, i would say, yes, you can use two different stylesheets depending on the case you expect.
To get the wording right:
You can only have one theme active at a time, but you could apply different templates to different pages (and that’s exactly what esmi states).
Peter
Thank you all very much for your comments!
