in myapp/context_processors.py:
from django.conf import settings
def debug(request):
  return {'DEBUG': settings.DEBUG}
in settings.py:
TEMPLATES = [
    {
        ...
        'OPTIONS': {
            'context_processors': [
                ...
                'myapp.context_processors.debug',
            ],
        },
    },
]
or, for versions < 1.9:
TEMPLATE_CONTEXT_PROCESSORS = (
    ...
    'myapp.context_processors.debug',
)
Then in my templates, simply:
 {% if DEBUG %} .header { background:#f00; } {% endif %}
 {{ DEBUG }}