First, there's a little CSS problem when choosing both small padding and a large border radius on the thumbnails. See screenshot. Been there.

But beyond that, I would simplify the border options.

I'd use a 3px border-radius all the time. Subtle, but good. Larger radii just make the page look old-fashioned.

I'd make the border and padding options simple check boxes. Choose border, you get a 1px border. Choose border and padding, you get a 1px border with 3px of padding. As with large radii, anything larger just begins to look like a relic from 20 years ago.

The drop-shadows are almost invisible. I'd set them at box-shadow: 5px 5px 5px rgb(0 0 0 / 0.25) ( rbga is now consider "legacy"). Still fairly unobtrusive, but gives the page a little depth.

There's also an inherent problem with the default thumbnail image bounds. If you choose Fixed shape for the thumbnails, and don't change the bounds, you get pano's. I'm not sure why the justified layout wants bounds that are at least 2:1 - I would think that 3:2 would be OK, and that would work for fixed-shape without needing any tinkering. The default bounds are 3:1!

Why am I obsessing about these things? Because I think the default skin should be as simple as possible, something a new user can use to make good-looking albums right out of the box.

I've also learned that giving users lots of options usually just means that you're giving them ways to make albums ugly. There are times when I'd like to go back and yank out about a third of the ones in my own skins.

The Captions and Lightbox options, on the other hand, I think hit the "sweet spot" perfectly. A few tweakable things, but never enough to destroy the appearance.

The problem with justified layout is you have to make all thumbs the same height. This is why it requires a wide enough horizontal bound. If using 3:2, all images having a wider aspect ratio will be zoomed, e.g. a 2:1 image will be zoomed by 33%. (Using 1:1 AR distorts images even more.) Perhaps this is still the better choice for an easier switch between the styles. I was thinking of adjusting the thumbnail dimensions automatically. I don't know.

I believe using the 90's-style grid layout as a default choice is not something new users will hook onto - even though I see your point. We have to trigger some wow effect right on the first make.

In the case of justified layout no FixedShapeFilter is applied, so with 300x200 bounds, a 2:1 AR image will result in 300x150px, which will be displayed as 400x200px. Not as bad as if the bound were 200x200, where the thumb would be 200x100px - displayed with a 200% zoom.

It's not masonry either, the height varies between rows.

I made the thumb dimension helper so it makes the thumbs 1.333:1 if AR > 1.5 for fixed layout, and 2:1 if it was 1.5 or smaller for justified.