I have created a custom theme and I wanted to use it on another instance of a shop. I have two instances (stage and dev). The problem is when I upload the theme I can't customize it. I'm getting a "404" page:
The page you’re looking for could not be found
Please make sure the web address is correct.
The main theme is based on Slate's default theme and it overrides the Debut theme. In order to make it work on the destination shop, I had to add a free theme (Debut) and override it using slate deploy command. I find it very confusing. Is this the only way to work with custom themes?
Here's how my themes.xml looks like http://take.ms/6ItNC
This is a year old, but I just had this problem, so I figured I would respond in case someone else finds this and is looking for an answer. My issue was with the settings_data.json file. It had previous/outdated data in it, which was causing a conflict somehow. I deleted everything from this file and the customize page worked. You likely could be more careful and just delete the conflicting pieces, but it was a new theme for me, so I was okay starting over with that data. Hope this helps.