Enabling a new themes sometimes changes the default theme

When I enabled a new theme (100days) that came before my default theme (minnelli) alphabetically, my default theme was changed to the enabled theme (marvin) that came alphabetically before my nominal default theme. I haven't looked at the code, but my guess is that there is an array of enabled themes that are numerically keyed alphabetically and that the default is set to a certain numerical key. but when that array is changed, the default theme doesn't change with it.