How do you access your website?

Oh, ok, I access my website only via websitename.com/administrator

In this way I enter the Joomla Backend, where I can change entries and so on.

OK, if you have the FTP access to your webhosting, then you can edit the k2.css file in the components/com_k2/css folder.

If you do not have the access, then i suggest contacting the person who has it.

If you're using templates like for example from YOOtheme, then from the backend you can goto the template area and modify the custom.css file

just add