Finally, i managed to solve the problem by droping the CSS file from the WORKSPACE_IMAGES and reload it again.
I also tried the same in apex.oracle.com with no problems, so i consider the issue closed.
On question, remains: if i want this to be included in every Interactive report i have, in order not to include the file in the page that IR exists, i supect i have to move this line to the template.
If i consistently use the template One Level Tabs - No Sidebar, where should put the line:Yes.
I guess in: Page Template | Cascading Style Sheet | File URLs.