Make sure that external_stylesheets style doesn't apply to the dev tools

[marthacryan]

See #3185 for an example of an issue that arises when external stylesheets are loaded and can override the css in dev tools

[ndrezn]

Can we ensure this only applies in debug=True?

[marthacryan]

@T4rk1n could you elaborate on why you don't agree here? Just for records / further discussion w/ Nathan (or anyone else that wants to agree or disagree)