See this thread
https://www.mobileread.com/forums/showthread.php?t=79367
In short you will have to create manually a folder called fonts for example in the root of your reader main memory and copy the arabic ttf to that folder.
Assuming your font is called Arabic and filename is arabic.ttf (just an example) you need to define this in extra_css field:
Code:
@font-face {
font-family: "Arabic";
font-weight: normal;
font-style: normal;
src: url(res:///Data/fonts/arabic.ttf);
}
body { font-family: "Arabic", sans-serif }