DEV: Fix flaky site.json api test (PR #14351)

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}]
...

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