internal: Improve how theme configuration is validated and merged. * Add default theme config up-front so it benefits from the usual validation. * Add an explicit `use_default_theme_content` setting. * Add/fix unit tests.

cov-core==1.15.0
coverage==4.0.3
invoke==0.12.2
mock==1.0.1
pytest==2.8.7
pytest-cov==2.2.1
pytest-mock==0.10.1