So 2.1 introduced a version tag for themes (and it's mandatory)
2.0 should in its next maintenance release include a check for this element - if undefined, do nothing, if defined do the version check.
We've had people breaking their sites trying to use 2.1 themes on 2.0... for a small backport+tweak of 2.1 code I suspect this would do a lot of favours as 2.1 moves forward.
(Antes, this one's your fault for making your fun WinXP theme experiment 2.1 only)