If you wear plain glasses correctly, they won't damage your eyes. Some cheap plain glasses may cause damage to your eyes if you wear them for a long time. So you'd better wear them at most 4 hours a day when you wear plain glasses for fashion. What's more, you should do some eye exercises often to relax your eyes.
No, you can feel ease to wear them and they will not be harmful to your eyes.But make sure there is no prescription on your lenses otherwise, you will feel uncomfortable or even dizzy when wearing it and your eyes will be adjusted to that prescription.