If you find that you can't install Grepper on Opera or OperaGX:
first download this extension from the opera extension store https://addons.opera.com/en/extensions/details/install-chrome-extensions/
then go to the grepper download page on chrome:
https://chrome.google.com/webstore/detail/grepper/amaaokahonnfjjemodnpmeenfpnnbkco

Finally, press "add to opera" and then accept the popup that comes up from the first extension installed

You can test to see if grepper is installed and working by searching:
"how to remove last element from a list python" and you should see the grepper box come up

If it's not working, search opera://extensions/ and give grepper permissions to:
"allow access to search page results"