TonyTomCat If you want a good template, I would start with these:
Ubuntu's Yaru theme: https://github.com/ubuntu/yaru
Pop!_OS's own Pop and Pop-dark themes: https://github.com/pop-os/gtk-theme
Note** to get Yaru and Pop to display properly in Budgie, you need to copy the CSS file from this github that a lovely user made very recently and simply put it in your home folder, log out and back in and you'll be good to go: https://github.com/speltriao/Yaru-Dark-Budgie-theme
You can edit that CSS file to change the values of the icon list indicator colors if you don't want it to always be the Ubuntu orange color, but it's all about tinkering and tweaking!
I say try these because all these are updated regularly, they are the defaults on some of the most used distros, and all included a working flatpak theme which is a huge plus for some users (including myself).
There is one I just stumbled upon while doing this that does support Budgie as well called AdwaitaExtended or perhaps it's AdwMod now, but it takes the Adwaita theme and makes it much more compact, which was my huge dislike for Adwaita how big it made everything and this one attempts to fix all that. Check it out here: https://gitlab.com/hrdwrrsk/AdwMod-theme
Hope some of this was helpful and good luck figuring out some themes!