Add default for default_syntax_highlighting_theme

What does this MR do and why?

The default_syntax_highlighting_theme application level has a database default, but the default is not represented in ApplicationSettingImplementation.defaults.

I came across this problem while working on !120853 (closed) which adds new users in a background migration. The tests failed because the new users didn't have color_scheme_id explicitly set. (color_scheme_id uses the application setting as default)

This MR fixes this by adding the default to ApplicationSettingImplementation

This MR adds the default on the application level so the validation does not fail for users created without a

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