After lots struggling, searching and researching, I finally found a working solution, which I am posting it here just for future reference to help out anyone who might want to do the same.
All I wanted was a simple way to use a locally installed external font file, preferably using a basic relative path.
Just create a new tiddler with whatever name you want, tag it with $:/tags/Stylesheet and add the following content to it, customizing where necessary
@font-face {
font-family: myFontFamily;
src: url(Font_File.ttf);
}
.tc-title{
color: red;
font-family: "myFontFamily" , sans-serif;
}
Adjust the src: url() part to a path pointing to your ttf otf or woff file, and adjust .tc-title to style whatever part of the wiki you wish.
Hope this helps anywome in the future