I've installed a third-party extension on a Magento 2 store for a client of mine and they would like some cosmetic changes making to it to make it match the site's look and feel.
I can edit the css file directly in app/code/<vendor>/<extension>/view/frontend/web/css but obviously if the extension updates then those changes will be lost.
What's the best way to change the css so the changes aren't lost. Should I just overide the css in my theme css or can I remove the extension css from the head and load my own version? Or can I copy the css to somewhere in my theme folder?
If you document those changes, then there should be no problem at all with the styles.
When you download a new version of the extension, you should simply compare the previous unedited CSS file with the new CSS file, and migrate the changes.