Back end changes problem

Dears,

I’m using Aegaeus theme and I have successfully imported the demo data.

The problem is that when I do any small change in the website design through the back end like updating the main menu items for example, the front end design totally got messed up (menu style is lost and homepage slider is missing,…).
A screen shot of the messed up design is attached after I tried to delete the main menu items under “Home” to only keep “Home” in the first menu item.
Please, advise.
Thanks.