To override the K2.css file so as not to loose your changes when upgrading K2 versions, place a copy of K2.css in your templates css folder and K2 will load it automatically.
You can also use more specific overrides as well by using css4k2 which will keep a copy of each of your overrides css in a file named YourTemplate/html/YourOverrideName_style.css
See Jiliko.net or k2joom for this free download