It looks like you want
app.ReadFile(path, "base64")
( not tested in css)
// test code
var b64 //global variable
function OnStart()
{
var path = "Misc/myfont.ttf"
b64 = app.ReadFile(path, "base64")
yn = app.CreateYesNoDialog(b64)
yn.SetButtonText("Copy to clipboard", "Cancel")
yn.SetOnTouch(yn_OnTouch)
yn.Show()
}
function yn_OnTouch(ans)
{
if(ans == "Yes")
{
var s = '"' + b64 + '"'
app.SetClipboardText(s)
}
app.Exit()