It looks very plain (black background, monochrome icons, very few apps included). You can customize all that if you want. I personally quite like the default appearance, but I am also the kind of person who uses the default GNOME or KDE theme on Linux and does not bother with custom themes or anything beyond daily Bing wallpapers.

There are numerous video walkthroughs of GrapheneOS. This would be one starting point:

<https://redirect.invidious.io/watch?v=aNgupWEV13M>

Visually, it generally looks much like stock Android in terms of capabilities, though a stock install generally has far fewer apps installed.

It's valid question for people unfamiliar with the project, but it is the AOSP in terms of looks, GrapheneOS does not customize the UI in any way beyond what their own features require as additions. Note that Pixel OS is not AOSP. The default home app of course also influences the experience quite the bit unless you replace it, which is what I'd personally recommend everyone to do as it's so incredibly barebones. Lawnchair is already a big step up as an open source alternative.

Perhaps screenshots and sleek UI is not their selling point (and it isn't).