Sounds great overall! Left a few comments here and there, none of them blocking/overly important, feel free to decide as you like.

The two more general (and more important) comments I have are:

1. I find it a bit odd that we're never listing all of the options in this guide, and are not even linking to where they are displayed on the config options page. I assume if I were a developer, I'd do the following thing: Google "Streamlit theming", go to the first page (probably this one), read the intro on how it works, and then look for a list of theming options that I can copy/paste into my config file and play around with. If I currently land on the page, I basically need to read through everything and then compile that list of options myself. (I totally get you don't want to simply replicate what's on the config options page, but then we should at least link to that prominently I think).

2. It would be great to show some images on the first page to make it clear what you can do! I know you're also working on adding examples and can totally be a follow-up. But would be great to make this very visual. We could at least have one image somewhere that shows a custom theme (similar to the old page), or even a small gallery of different theme examples that you can click on to view the apps.

Some spelling mistakes and a few comments.
When I ask a question, I don't usually need an answer; I just want to call your attention to a possible issue for users.
As always, let me know if YOU have questions!