While customizing my blog with CSS upgrade, i found out that the box on the left displaying date and comments had been changed ( for instance th arrow on the right pointing at the post is gone )… And i don’t know which part of the original CSS i should go back to.
Can someone give me an advice?
You have pasted the entire stylesheet into the CSS edit window, and at wordpress.com that can cause issues such as all theme-related images disappearing. That is because image URLs in CSS are typically “relative.”
At wordpress.com, the best way to do things is to add only the selectors and only the specific declarations you are adding to changing to the CSS edit window and then make sure that “add to existing…” is selected.
The other choice would be to go in using something like Firebug add-on for firefox, find all those relative URLs where the image is missing, determine the absolute URL and the replace the relative with the absolute. This would be an example of the absolute URL for one of the missing images.