You have to compare version of your theme with the current version (Dashboard -> Storefront -> Updates)
Here you can find link to theme updates, click "download" to download the latest version
Changelog are placed in the Updates section:
https://www.gavick.com/updates?task=upd ... 30&catid=4so you have to replace all listed files.
Generally if you've changed only 3 css files, maybe will be easier to add your modifications again (this time try to use override.css file).