In version 8.3.15, MiMix2 can not apply the themes that were uploaded to the internal storage. It allows selection but no longer appears in front. This is just me or you've experienced it already.
I've downloaded Stabil 5.2.0 and I've it works Themes from an external drive. TWRP - Dalvik - Cache - System - Data deleting. Thanks for the idea of this success !!