[vishcompany]

It works well here.

1. You add the fonts of your choice to the mentioned folder /fonts. Preferably each font comes in 4 variations (4 separate files): regular, bold, italic, bold-italic.

2. You add a .css file like the following example into this folder:
.../system/PRSPlus/epub

Example: fontin.css

@font-face {
 font-family: "Fontin";
 font-weight: normal;
 font-style: normal;
 src: url(res:///Data/fonts/Fontin-Regular.otf);
 }

 @font-face {
 font-family: "Fontin";
 font-weight: bold;
 font-style: normal;
 src: url(res:///Data/fonts/Fontin-Bold.otf);
 }

 @font-face {
 font-family: "Fontin";
 font-weight: normal;
 font-style: italic;
 src: url(res:///Data/fonts/Fontin-Italic.otf);
 }

 @font-face {
 font-family: "Fontin";
 font-weight: bold;
 font-style: italic;
 src: url(res:///Data/fonts/Fontin-SmallCaps.otf);
 }

 body {
 font-family: "Fontin"; 
 } 

 p { 
 font-size: 1.0em !important;
 text-align: justify !important;
 line-height: 130% !important;
 text-indent: 1.5em !important;
 margin-top: 0.5em !important; 
 margin-bottom: 0.5em !important; 
 }

 html { 
 font-family: Fontin !important;
 }

 .mainBody {
 font-family: Fontin !important;
 }

 .calibre {
 font-family: Fontin !important;
 }

Of course you have to adapt the file according to the used fonts. If you don't like the line spacing, you can change it with "line height".

Watch out, when editing the file.
Avoid using the regular windows editor (if you are using win), it can mess up the file. I use Notepad++. It's free and works great.
Avoid spaces in the filenames.

When everyting is done, unplug safely, choose the new font, which should now be listed in the appropriate PRS+ menu, open another book first and then your actual book again.
Enjoy.