`Highlight Ads on Google.com` works in a very simple manner: it injects a style sheet to google.com which locally applies a background color to search results that are ads.
After you have installed this app, the extension will appear in Safari > Preferences... > Extensions . Here, you can turn the extension off and on - or uninstall it.
If you have any problems, contact me at ulrik.lyngs@gmail.com or create a GitHub issue on the repository github.com/ulyngs/highlight-ads-on-google/.