The color_scheme_id needs to be an integer not a string.
This is one of the failing tests that showed this error:
https://github.com/discourse/discourse/runs/3598414971
It showed this error
`POSSIBLE ISSUE W/: /user_themes/0/color_scheme_id`
And this is part of the site.json response:
```
...
"user_themes"=>[{"theme_id"=>149, "name"=>"Cool theme 111", "default"=>false, "color_scheme_id"=>37}]
...
```