Can't add CSS anymore

Hello,
I can’t add custom css anymore to my site. I mean, I can add it, but nothing happens anymore after adding custom css. (tried several custom css code, even the very simple ones, but it looks like custom css is not accepted anymore).

Maybe somebody can help me with this?
(i.e.: I tried this code to give the backgroud color of the tabs another color, but It’s not working?

.active_tab_content, div .main_color .tabcontainer .active_tab {
background-color: #E0E0E0;
color: #666666;
}