pls help 
Partly due to the fact that the game is in development and 1:1 compatibility with SM5 themes were largely removed, as well as the fact that nobody these days is brave enough to learn, no themes exist for Etterna.
At least they wouldn’t if I didn’t try to keep some alive.
You can find a link to spawncamping-wallhack on this forum. I maintain it and make releases for it on the main repo and in my own fork. Currently, the most up to date release on my fork is the only version that will work on 0.70.3: https://github.com/poco0317/spawncamping-wallhack/releases
I also recently made an empty example theme called bare-frames and shipped it with the game in the latest version. It seems to not work for almost anyone due to a mysterious crashing error. I believe the solution to this is to open metrics.ini and toggle AutoSetStyle.
There are 2 themes in development, and I intend to release Etterna 0.71 with another theme alongside it. I do not know the progress of the other theme. There are also private themes that exist out there if you like to play on old versions of the game and perhaps eventually have your scores be invalidated.
I will rarely convert themes, with very minimal effort put into it, on request. Usually an hour of time at max is dedicated. Updating a theme from the anemic featureset of SM5 to what Etterna offers is unfortunately both an annoying and boringly long process which will take months in any case because I really really don’t want to waste my time doing it.
are there any useful resources you’d recommend for learning more about etterna themes (other than just reading through the existing themes to figure out how they work)
I actually learned how to do it by studying both the Lua and C++ as well as Til Death, so I do not know of the usefulness of guides on the topic or how many exist at all. Within the realm of Etterna, as long as it does not pertain to noteskins (although that works the same and I can probably figure it out if needed to) if you ask a question on the discord I usually have the answer quickly. You can also use other SM communities for help since the main difference, as long as those communiuties have abandoned the cmd() standard and xml format, is the set of functions we have added or removed. No list exists of those, but may soon.
This page provides a fair bit of good info which is still relevant to us: https://github.com/stepmania/stepmania/wiki/Theming
Reading that and using bare-frames as a guide (if you can get it working) might be good to get you at least an understanding. You can also of course use any functional theme as a guide. My way of learning was literally “change this thing and see what happens” and I have done that for countless hours just to figure out how the game works if the code doesn’t tell the whole story well enough.
The issue with the guide is that it mentions cmd, which I would avoid at all costs. It is not functional in Etterna. Another thing is that the guide has a dead link to a very good explanation about Actors, the base object of literally everything in the game. It also contains general info that is just very useful. It’s a good read. Here is the correct link: https://quietly-turning.github.io/Lua-For-SM5/Introduction/Foreword