Proposal: Force all themes to use lowercase names

I've noticed that the themes using an uppercased name are missing out on tab completion (unless uppercase letter is specified) or even just basic colorscheme name cycling. This would be an easy change but would force existing users to update their vimrc.

The issue does not name specific files or tests. Inspect the repository's theme naming conventions and determine which names must become lowercase, then verify that theme completion and colorscheme cycling work with the updated names while accounting for the stated vimrc compatibility impact.

Written by the indexing model from the issue text.