Assuming you mean this slider on the homepage: 2022-02-22 14.29.25 vivaldi.com 17b05c20a8d3.png
This is not a theme or anything like that, but simply certain settings are turned on or off to get Vivaldi to look this way.
Here, we see that this user's Vivaldi layout has two-level tab stacks turned on, and they have also clicked and dragged from the address bar to show the tab thumbnails (make sure this option is turned on in settings).
They have their Bookmarks Bar turned on and appearing directly underneath the Address Bar.
They currently have three tabs tiled; the one in the upper left is the Vivaldi Blog page, another is their Speed Dial page, and the third one on the right is a shopping website.
Something else to note is that they have their Panel set to the right side of the window rather than the default left side.
Something to note about the slider on the main Vivaldi page: it is not actually very representative of the custom feature sets available. Most of the "different" options are really just simple variations or one-step basic tweaks, but to the average user it seems like there's a lot of ways to use Vivaldi. Now, I'm not saying there isn't a lot of ways to use it, because there's a ton. But this is a poor representation of what you can actually do, because it only showcases a few, not particularly unique features of Vivaldi. I suppose two exceptions would be the tab tiling and the two-level tab overview, but if you look closely, you can see that most of the image changed by the slider are really simple tweaks.
Images 1, 2, 3, 4, 5, and 6 are showing layouts of Vivaldi that are all essentially identical. 1 shows a plain layout with no address bar, no tabs, no visible Panel or Status Bar, and a simple color palette. 2 shows the same version but with the traditional address bar and tabs turned on. 3 shows the same things as 2, plus the Panel on the left side, the Status Bar on the bottom, and also a minor tweak that forces Start Page to only show 4 thumbnails per row at maximum. 4 shows exactly the same layout as 3, but with a different theme. 5 shows the exact same layout as 4 except that the user has opened the Bookmarks panel on the Panel. And from 5 to 6, it's the same case except that they have moved their tabs from the top to the right side.
You can look through the rest of the slider to see the rest of the similarities yourself, as many of the other images are only slight tweaks to the layout, or non-layout-tweaks altogether.
As a Vivaldi user, I understand why they chose to do that, to make it seem like there's a lot of change that you can control quickly and thus another reason to switch browsers, but that doesn't mean I have to like it. I personally wish they showed a much more in-depth selection of images, showing how you can make major tweaks with the custom CSS and all, among many other things, but I do get how simple things that appear vastly different (with the key being that the average user already understands the changes).
Hope this explanation made sense.