as of now we don't really have an answer for that question.
We are evaluating a couple of options, namely, providing an alternative stylesheet that uses image icons or employing svg icons, with both having their ups and downs.
The ups for image icon alternative are that images are unlikely to be disabled as technology anytime soon, unless you are browsing in windows high contrast mode with a browser lower than Edge. The downs are that we need at least 3 sprites for the task and a workaround for the aforementioned high contrast mode.
The ups for svg is that it doesn't suffer from the high contrast issue; it's scalable; controllable by css (if made the correct way)... The downs are higher markup payload; couple of browser issues to name a few.
As soon as we have chosen a direction, we'll update this thread with our intended resolution.
Telerik by Progress