Summary: Unlike the desktop site, all mobile main sites (except SO and its language variants
and A51) have the same blue theme and all mobile meta sites (except bugged ones) have the same black theme. However three "bugged" mobile meta sites erroneously carry out a blue theme.
As a mobile web user of the Stack Exchange for over a year, I'm familiar with the web page designs.
When I'm visiting Android Enthusiasts, Super User, Unix & Linux, I get this blue theme
When I'm visiting Meta Stack Exchange, Meta Stack Overflow, English Language & Usage Meta, I get this black theme
When I'm visiting Software Recommendations Meta... Wait, it's blue???
I'm sure this is not my browser's bug, but I don't mind telling you it's Google Chrome for Android version 59.0.3071.125 from Google Play.
Some blue metas (
this list may be incomplete):
Edit: I finished searching. These are the only 3 bugged meta sites (as of July 8, 2017)