Thanks to barium for info he posted in a few threads (
here and
here), adding the following to my custom .css files on my reader with PRS+ installed so far has always overridden embedded fonts in specific epubs for me. Change LexiaDaMa of course to whatever font you're using. Before I discovered PRS+ I would use Calibre to redo the epubs to embed my favorite font, and as a test I had one book with about 5 different fonts embedded. All of them get overridden with this extra code. Barium has more code that he uses that you might find helpful too, but this is the minimum to force your own font from your reader.
Code:
html {
font-family: LexiaDaMa !important;
}
.mainBody {
font-family: LexiaDaMa !important;
}
.calibre {
font-family: LexiaDaMa !important;
}