Thanks!
I found this thread …
https://discourse.getgrav.org/t/included-grav-logo-svg-missing-on-extending-theme/5413/5
Then visited here …
https://github.com/getgrav/grav-theme-quark/blob/develop/quark.php
Where rhuk wrote: “… use Devtools plugin to create a new ‘inherited’ theme. It does most of the work for you.”
I read …
https://github.com/getgrav/grav-plugin-devtools/blob/master/README.md
So, do I create a new theme (a copy of Quark with a different name), then paste my yllix ad script in at line 20 of ~/quark.php (the “public function” section)?