Theme "isis" back to the default alone

Hello dear colleagues,

I have a problem that had never seen. I have a website in wordpress 4.1.1 to use the theme isis, which made several changes in his css to meet my need.

However, every day, the site redisplay the theme isis without the changes he had made. To resolve provisionally, I restore the backup. But it turns out that this “solution” is nothing viable.

Thus, I ask everybody to help find a definitive solution, and if you need more information, just talk.

I thank the attention and help of all.

PS: Site: http://lapidando.com.br/