To start, I wanted to show how inclusion of various CSS files could be used to change aspects of a site design. I made a very rudimentary example that queries the server to get the date, and returns a CSS file via import based on the current month. There is one basic stylesheet, and twelve possible imported files. The actual example is nothing fancy, just a proof of concept.
I have replaced the name of the imported CSS file with a short snippet of PHP, which in turn generates a numerical value (1-12), and appends the *.css file extension. The implication is CSS that self-updates throughout the year.
The code listed above is all that it really takes. The server is asked for the current time and then returns a numerical value between 1-24 for the hour and 00-59 for the minute. This yields a total of 85 images used, with one for the clock itself. The corrosponding ID is then updated with a new background. I hope these examples have proven helpful to you. Feel free to reuse them.