I purchased and use 6 of the SMF Packs but it gets to be a bit on the frustrating side when any of these mods have updates released.
The fact that updates are released fairly regularly is great, but from an installation point of view, I spend a lot of time having to compare the latest CSS files included with any releases to the customised CSS files that I use with these mods and then update the new ones accordingly.

Would it be possible to have some way (maybe an admin setting in each pack ) that enables us to specify a CSS file which contains over-rides for the one supplied with the pack? This would save me (and I believe others who use custom themes) heaps of time when doing upgrades of SMFPack mods.

I think I (almost) never change the CSS of my mods, you should be fine just doing a backup of the css files and then restoring the. If something isn't working property then you can use a diff tool to compare latest vs installed and see if the css files were touched.