Hi, you can find general info. on themes and links to themes for your 5G ipod here:
http://www.rockbox.org/twiki/bin/view/Main/WpsGalleryThe direct link to the iPod 5g gallery is
http://www.rockbox.org/twiki/bin/view/Main/WpsIpod5gPlease see this page for information about WPS pages on your iPod
http://download.rockbox.org/manual/rockbox-ipodvideo/rockbox-buildch9.html#x12-1960009.2.1Some themes require patches to the firmware. They will tell you this. I would avoid these themes and dealing with patches until you are comfortable with the general usage and updating of both the rockbox firmware and themes.
In general. Download the themes zip file to the root of your iPod and unzip it. If the theme is packaged correctly the files will be placed in the correct places in the '.rockbox' directory. If you don't see a '.rockbox' directory, you can view the directory by turning on the "view hidden files" option of your Operating System's file browser.
If you end up with a WPS, Theme, and/or Font directory in the same directory as the zip file, you will need to manually move the contents of these directories into the corresponding place in the '.rockbox' directory.
Rockbox can seem a little complicated at first, but it's just because you are trying to do 20 things to get it up and running initially. Read the manual, search the forums (99% of any questions you have have an answer lurking in the depths of the forums posts), and then post any questions you still have.
Don't be offended if all you get is a pointer to a page with a reminder to read the manual.
I put rockbox on my iPod nano two months ago and haven't used Apple's firmware since.