It mostly depends on what the negative reaction is, and whether you have any part of the userbase that likes it.
Because the reality is that what you need to do is encourage the supporters and let them be your advocates and cheerleaders - they'll bring along the rest. And if it's just about 'oh it looks different and things aren't where they used to be', people get used to that. Assuming the theme isn't eye-gougingly awful, people usually get over 'oh it looks different'.
The biggest kicker is when functionality people depend on, and often that you don't know about, goes missing, and with no sign of returning.
If there aren't a decent number of people who can be your cheerleaders, oftentimes the best thing to do *is* to go back. Just be honest with your memberbase - "we thought we'd make this move, we thought it'd give us xyz benefits, but it turns out it didn't work for us" - and usually people get it. But that stance rather relies on there being tangible xyz benefits that you think will benefit the community; I've seen too many cases of 'that site is popular, if I use what they use I'll be popular too' which isn't true.