In this case, it really is just minor stuff; Things that most folks don't notice, like fixing a parenthesis to ensure proper calculation of dimension multipliers (actually I think that's all base UI.X and UI.X black had this time around), or little quirks that come up in edge-cases that are rarely encountered.
Now, for the UIX & AC.UI Variations theme versions, I did note a significant fix - The default search node icon wasn't rendering, and that was noted in the changelog (so folks can remove their fixes if they had one, or potentially change how they implement their own icon if necessary). Additionally, it's noted in their changelogs that Xenith & Abyss both had the header overlay color changed to a specific RGBA color rather than using color functions to make it transparent (the function in which was broken in a recent XF release).
