These themes are focused on reducing whitespace for all screensizes, make comment chains easier to follow, and aim to provide some variance from the default themes available on Lemmy.
These themes can be used with any extension that lets you inject custom CSS (such as Amino for Chrome and Edge).
For people making use of Stylus you can install the userCSS by going here, and choosing your theme and color scheme of choice in the userCSS preferences. Stylus makes themes like this a lot easier to use.
I've recently fixed some issues, and created a bunch of dark themes as well. Feel free to log any issues on the repo (including requests), and feel free to submit pull requests if you have any themes you've done that you'd like to add!
Just a note on light vs dark themes, the light themes are designed to work with the "Litely" theme in Lemmy's settings, and the dark themes are designed to work with "Darkly", so make sure to choose the applicable theme in your lemmy settings before applying one of the themes on the repo.
Special shoutout to @communist for the Ancom theme!
Here's an example of the yellow dark theme:
EDIT: A note on Stylus, currently the userCSS only applies to beehaw.org, but you should be able to edit the theme to change which domains it applies to, for other Lemmy instances.
Honestly, I have no idea; can't say that's happened to me, but it might be best to post about it in /c/support so the admins see the issue and can potentially look at a fix (if it's not already been reported to the Lemmy devs).