Now that WOULD be cool and it would justify the apocalyptic look. I like your forward way of thinking.
Thinking off hand about this, I'd say the best way would be to do a JavaScript (MooTools most likely) selection of CSS (or in RT's methods CSS directories like style1, style2, etc.) files that would determine the look based upon the day according to either the server's date/time or the client's.
var dSS = {
names: ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"],
number: new Date().getDay(),
link: document.createElement('link')
}
dSS.link.rel = "stylesheet";
dSS.link.type = "text/css";
dSS.link.href = dSS.names[ dSS.number ] + ".css";
document.getElementsByTagName('head')[0].appendChild( dSS.link );
This is just something quick and rudimentary, you could definitely devise something that is more like Weekday.css and Weekend.css so you do not need a CSS file for each day of the week.
Also do not forget to set up a way for pleasant degrading:
<noscript>
<link rel="stylesheet" type="text/css" href="whatever.css">
</noscript>
Also, I did not do any work considering the folder layout of styles by RocketTheme, again totally doable.
Hope that helps you get on the right track. Definitely check out MooTools if you have not already, and even ask about solutions at their forums. They are good people and will definitely help you learn more about MooTools and JavaScript.
Good luck
Jeff