Forgive me if the answer to this is stunningly obvious and I am just being unobservant.
I cannot activate several of the 'Simple' themes because it is missing the parent (named just 'simple'). However I cannot find this in the themes section for download. Is this a version issue or something else? I am running version 2.1.6.
[extension by saimon] Simple Grey
I agree it's not obvious. You have to install "Simple Grey" ("simple" is its unique identifier). In the roadmap, we have an improvement on plugins/themes installation : automatic install of dependencies.