I like the white version better for some reason! Maybe because I am expecting something complicated, it would be nice that everything is clear.

I know the limitations of Marvel and it sucks that you can't implement animations sorta, but I am looking forward to the actual product!

I couldnt help but notice that the placeholder text of the 'Paste your URL' is centered, but in the rest of your input fields it is left aligned. I would keep this consistent and have it left-aligned aswell :)

I love the way you display the title and first sentence of the post you just want to get upvoted. However, I would consider switching the locations of the 'Post found' and the preview. In the very popular Material Design as well in other Design, an error considering an inputfield is commonly added below the field rather than right next to it.

At the bottom, it is quite weird that you first show the link, and then the description which ends on a :. Since it's the 'conclusion' of your page, you might want to make this more obvious. I am thinking maybe you could just center the whole section with the text on top of the memotext.

I really like the design. It looks very clean and simple. Good luck :D