Alright, this is about to drive me crazy. I guess there are other ways I'll try theming but this bugs me:
... I edited the header.tpl of the Vanilla theme, and after all these steps it only shows me the header.tpl of the Vanilla theme (instead of Banana)! What am I overlooking? 
@sdnyco Did you restart NodeBB after you made the changes?
I wrongly used the term reboot instead of restart before–but yes: I did restart nodeBB.
@sdnyco message me so we can chat on Skype because this is going to drive me nuts 
I'd be more than happy too. I'll message you in a sec. 
I had the same problem, but after looking through the code I found the solution...maybe you have as well, but I thought I'd put it up here for others.
When you clone the nodebb-theme-vanilla folder you also have to add this line to your theme.json file:
"templates": "templates"
Should work fine after that. Basically, what happens is when nodebb loads it looks for that line in your theme.json file and if it doesn't find it it defaults to using the vanilla theme templates. So, even though it might be telling you your theme is loaded, it's not...kind of confusing.
@mootzville You are my hero. That did the trick! 
Hats off to you, Sir.
Now on to actual theming!
No worries @sdnyco. Enjoy!
@mootzville Thanks a lot for that, I'll keep this in mind for future issues!
Thanks for reporting the bug @mootzville, the templates property is no longer required in a custom theme's theme.json in v0.5.1 (but since we haven't even launched v0.5.0 yet, don't worry about a thing, the fix is backwards compatible)
