Before upgradingg, did you make certain you had a copy somewhere of the formal white theme as you customised it? Because when upgrading the original theme is of course restored, or copied back if you like, so all files are replaced again by the original ones. So after that you have to overwrite it again with your altered version. Best way to adapt an existing theme is therefore to make a copy of it first, which you rename then (e.f. mytheme), and select as theme (in configuration). Then you apply all changes to this newly created copy. And of course store a copy of the customised theme someplace other than in your themes folder. That way your alterations are not overwitten when upgrading, and you have a backup copy anyway to put back in case something goes wrong.
you can read about the work with themes in the Themes Documentation in the Wiki. You can read there how to build your own theme based on a standard theme.
Ah, I wonder if this is the same problem as I am having - (which I posted separately not having seen this post *doh*) in which I appear not to be able to change colours in the theme... I think I have followed the guidelines correctly for changing it.