Unless you're working on a ready-made template, you should be fine adding these into your main template.css file.
Even better, you can override K2's CSS directly.
Just copy the file /components/com_k2/css/k2.css to /templates/YOUR_TEMPLATE/css/ and then change as you want. Doing so will allow for any K2 CSS customizations to remain intact even if you update K2.
Fotis / JoomlaWorks Support Team
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Please search the forum before posting a new topic :)
