I think you were pretty close
@font-face {
font-family: 'MarkPro';
src: url("_/theme/markpro.otf") format("opentype");
}
@font-face {
font-family: 'MarkPro';
font-weight: bold;
src: url("_/theme/markprobold.otf") format("opentype");
}
you missed the quotes on font-family and using a relative url means that it works in the ide and in the live app.