oops - I didn't mean to hit the "heart" button on that one. My feeling is that these types of conversations are counterproductive. That being said, I don't happen to believe that Clinton "Stole" the California primary or any other primary in the sense of altering voting machines and the like. I have looked into the so-called data on this that involves exit polling and found nothing. For some reason, people seem to constantly misrepresent the purposes of exit polling and what you can infer from them - I often try to make that point when I teach this stuff in my Statistics courses.
However, I see three important points here:
1) it is true that the DNC was not the neutral party they claimed to be throughout the primaries and that the scale was tipped against Bernie in a thousand ways inherent in our system. (of course Bernie knew about those types of obstructions and built a tactically smart campaign under that knowledge and nearly won.)
2) The DNC continued to publicly deny that they were favoring the Clinton campaign over the Sanders campaign again and again. Thus, when the truth came out about that, they had effectively poisoned their own well - exactly the atmosphere in which conspiracy theories grow.
3) People on the left who spout those theories believe them because of that poisoned well. Thus, calling them disingenuous isn't quite fair - any more than calling you disingenuous for your beliefs.