Hello, Ziping,

The provided video is greatly appreciated.

I was able to observe the same undesired behavior on my end and confirm there is a bug with setting the font size while creating a custom style. This is why I have logged it in our feedback portal by making this thread public. You can track its progress, subscribe for status changes, and add your comments on the following link - feedback item.

I have also updated your Telerik Points as a  reward. 

Make sure that you follow this item in order to get notified when the status has changed. We will do our best to introduce the fix in our upcoming R3 service pack release. Stay tuned!

Should you have other questions do not hesitate to contact me.

Regards,
Nadya
Progress Telerik