From the comments fo @OneTrickPony, I came to know that,
Mystique hasn't been updated for a long time, but a major update is going to be released these days, and most likely this won't happen in the new version...
So, I conclude that issue is coming from the Theme itself. ( Not sure, but from the comments of @Milo & @OneTrickPony )
So, I uninstalled & removed mystique theme & at last I installed other theme. I know this is not the exact solution. But I had no option.