Hover/focus style for patterns, especially on dark backgrounds, has artifacts

[jasmussen]

This is in nit-picky territory, and it's probably related to #128 (comment) and how hover/focus styles are drawn on thumbnail previews.

But in case a refactor allows us to improve things, there are small edge artifacts on thumbnails. It's not immediately visible:

But if you zoom in, you can see the white artifacts:

This is likely from the focus style being stacked on top of a white border, using a pseudo element, rather than being drawn on the parent instead.

[ryelle]

Moved to #128.