Some skins give you the ability to change the font, Decent skin is one that does I think.
Not sure about the Dimka skins though.
Otherwise it has to be changed in the DATA.ZIP file.
Get your desired fonts, rename them to exactly the same as the fonts are named inside DATA.ZIP\ui_igo9\fonts folder.
Replace the originals with your new ones.
Delete the Save folder (back it up first if you want to keep any saved tracks etc.) from the root directory before running Primo again.
Before you do anything though, make a complete backup of your working Primo so if it breaks it can simply be replaced with the backup.
Changing the font sizes can be more complicated, unless there's a sys.txt entry that covers it I think the sizes for various font displays are changed in the template.css, the .ini, the .ui and perhaps other files in the various folders (800_480, common etc.) within DATA.ZIP
If using a skin the ones inside the skin's relevant folders might affect things too.
Possibly also .vpr (polygon co-ordinates) files depending on what it is you want to change, I've had to change some of them in iGO8 & Primo skins to get the desired results, though the ones I've changed have been for small image-like displays on the screen like POI direction arrows & letters on a small circular compass etc..
Don't unzip/rezip your DATA.ZIP & don't work on it while it's on the device.
Copy it to the PC & work on it there, then when finished delete the one from the device and paste the edited one back to the device.
Depending on what I'm wanting to change I can make up to 3 copies of DATA.ZIP or a skin file when making changes, one to keep as a working original in case I break something, one to make the changes in & paste back to the device when all changes are made and the 3rd to actually unzip (discard when finished) to find any images or other files I might want to change, which is probably the quickest way to find them.
Use WinRAR version 4 or lower and I think Total Commander also works, once you're ready to replace something open the DATA.ZIP in WinRAR, replace whatever it is by dragging and dropping where it's supposed to go & just click ok, yes etc. when it asks.
This can be done without actually un-zipping, but working within WinRAR itself.
Once finished & saved etc. paste it back onto the device replacing the original, by first deleting it not overwriting.
If I remember I usually rename the working original on the device itself to OLD_DATA.ZIP or something similar, then if I break something or get sick of trying to do whatever I'm doing and give up, it's just a matter of renaming it back to DATA.ZIP & removing any that failed to get a working Primo again.
Backgrounds could be in either the skin or DATA.ZIP files, trial and error and experimenting with those, sometimes you can work out what they are by what they're named, other times it's not so easy.
They're mostly all compressed .BMP files, use a program like iPAQ31x to browse the compressed files with and also uncompress them so they can be worked on if required.
I usually uncompress them to .png files as a lot of them have transparency, work on them with GIMP, save when happy with changes, then use iPAQ31x to recompress them to iGO's compressed .BMP format.
Replace any files in skins the same way as DATA.ZIP, do not unzip/rezip but open within WinRAR, replace anything, then ok, save etc..
Good luck