You really should read the wiki.
http://www.rockbox.org/twiki/bin/view/Main/CustomWPSHowever, I will punch through it for you briefly.
Basically, the following terms will explain what they do and where they go:
-Configuration files - Tells rockbox what files exactly to load. Goes in /.rockbox/theme
s/yourthemename.CFG (File Type MUST be cfg, and make sure the folder is themes, not theme)
-WPS - Well, we know what this is. The while playing theme. The *.wps goes in /.rockbox/wps/yourwpsname.cfg, and the images and such go in a folder in /.rockbox/wps/yourwpsname (folder must be EXACTLY same name, I think case sensitive, as your *.wps file.
-Backdrops - Shows an image as a backdrop while in menus and such. Goes in /.rockbox/backdrops/yourimagehere.bmp (Must be exactly the size of your screen, refrence to the CustomWPS wiki page linked above.)
-Fonts - Displays a font for use in the WPS and in menus. Must be /.rockbox/fonts/fontnamehere.fnt (You can download the newest build of fonts off the daily builds page, or you can make a font for yourself using any program that converts fonts into *.bdf and then into *.fnt, or directly into *.fnt for rockbox.)
Here's a quick explanation of the line that you posted about.
%?mp<|%xdk|%xdl|%xdm|%xdn>
%?mp means playback (Stop, Play, Pause, Fast Foreword, Rewind, in that order).
%ac%cl:M pc
%ac means centered text. %c means start time display. Time displays are ended with the letter c, and everything in between the %c and the c at the end is display information (What you want to be displayed). For example, %cl:M pc means that what's going to be displayed is going to be centered and show Hour:Minute PM (uppercase).
Though my configure is for 24hour display I only get 12hour displayed with this theme. I suggest this command overrides the configure settings. Am I right?
No. Not at all. Even if your rockbox menu time is made to show 24 hour time, the wps doesn't tell you the time normally, unless you either have the status bar enabled, or if you tell it to in the wps. To show 24 hour time, change the l to either a "k" (if you want to display 0-23) or "H" (if you want to display 00-23) on your wps.
%ar%?mh<%xdw|>
%ar means align right. %?mh<%xdw|> means that if the hold switch is on hold, it's going to show a hold icon that is preloaded.
%?bl<%xdo|%xdp|%xdq|%xdr|%xds>
%bl means Battery Level. Meaning, this line tells the wps how to read the battery through the wps. That's all.
Hope this is helpful.
-Dan