is it feasible? Sure. The script's CSS can be essentially copied wholesale into a required stylesheet.

Will it ever happen to the "Default" Lemmy UI? Probably not.

There are a few catches with this: 1) it has been explicitly communicated by the Lemmy devs that the stylesheet and styling framework (Bootstrap v4 / v5) will be changed in the future for a different framework, and 2) classes and IDs used by elements are not considered "safe" or "static" for future development. These issues are less of a challenge for traditional theming, which applies different colors to standard web elements; but they are more of a challenge for things like this that make wholesale changes to the layout (and largely leaves colors alone).

That said, there is at least one instance that I am aware of that is trying to make this the default layout: forum.basedcount.com. It appears that incorporation has not been rolled out just yet though.