TypeError when css_files is None with Sphinx 1.6 #160
Description
Using Sphinx 1.6.1, running quickstart, then configuring sphinx_bootstrap_theme according to the README, the docs fail to build with this error:
Exception occurred:
File "/Users/jaraco/.envs/sphinx/lib/python3.6/site-packages/sphinx_bootstrap_theme/bootstrap/layout.html", line 40, in top-level template code
{% set css_files = css_files + theme_css_files + bootswatch_css_custom %}
TypeError: unsupported operand type(s) for +: 'NoneType' and 'list'
This issue emerged as part of the Sphinx 1.6 release. Pinning to Sphinx 1.5.6 works around the issue.