You can create a custom .typ file that alters the appearance of the map - see this thread for starters, however the website mentioned there no longer exists: http://www.laptopgpsworld.com/5084-backroads-gps-maps-different-software
Here's another thread on the topic, although it refers to the Nuvi. http://forums.gpsreview.net/discussion/27990/better-looking-topo-maps-for-the-nuvi-3xxx-and-others/p1
HOWEVER, there is one big caveat on MobilePC: bitmapped types do not work properly. Lines will display erratically and polygon fills will contain garbage. This is a big limitation of MobilePC unfortunately. But you can use regular types on MobilePC - bitmapped types use an image to create lines and polygon fills. So a bitmapped line could be dotted or dashed and a bitmapped polygon could have little trees in it. Regular types just allow solid colors.
For editing .typ files I currently use TYPviewer: https://sites.google.com/site/sherco40/
For Garmin's newer 24k maps, you might need to first convert them to the old format with MapReverseConverter. I remember doing this for mine, but not sure if it's necessary now since some recent tools allow you to change .typ files for the new maps (gmap format).