Something like this: Bullet Hell Jam 2023 - itch.io
Is this one just exceptionally good, or am I missing something in my creation?
Ah, yes. Looks like that one has custom CSS enabled. You must contact support to get that. However, there’s a reason why it isn’t allowed by default. It is possible to make very good pages without, and CSS can be abused.
I don’t know what your jam looks like, but I have a feeling it’s more to do with that than any lack of CSS.
I came here looking for how to add background images (to the whole page, not the content area). From your post, it looks like you can. I don't have that option. According to @mid, it looks like I might have additional options by enabling CSS.
However, I would like to mention, you can use custom HTML. Something as simple as adding a background-color (to the html), border, border-radius, color, bullets, numbers, sections, headings, etc., can all make your page look nice.
