Hi Kanaat

Any non standard content or scripts will be deleted by the editor to prevent malicious content being inserted.

I think the best way to apply CSS to a specific block would be to add the CSS code as Custom CSS under Site admin > Appearance > Themes > Basis

Apply the CSS code to the block instance for example if the block id is 9 then the CSS can be applied to just that block as below

#inst9.block_calendar_month .header{background-color: #008aaf;color:#FFF;}

Regards