I want to add my custom font to website theme editor > options > font family. That custom font isn't related to "Google Web Fonts" and it has its own folder and files. I'm using Odoo 14 with default theme.

You can give a path of your font folder in css.

.test {

        @font-face {

        font-family: 'myfont';

        src: url('/theme_test/static/src/fonts/230163575-DINNextLTArabic-Regular.eot');

        src: url('/theme_test/static/src/fonts/230163575-DINNextLTArabic-Regular.eot?#iefix') format('embedded-opentype'),

        url('/theme_test/static/src/fonts/230163575-DINNextLTArabic-Regular.woff') format('woff'),

        url('/theme_test/static/src/fonts/230163575-DINNextLTArabic-Regular.ttf') format('truetype'),

        url('/theme_test/static/src/fontss/230163575-DINNextLTArabic-Regular.svg#DIN Next LT Arabic') format('svg');

        font-weight: normal;

        font-style: normal;

        }

            li {

                font-family: 'myfont';

                }

}