merge powerarrow-dark theme as branch of powerarrow

These two themes appear to be closely related and share quite a bit of
code. Maintaining them as two separate projects seems like a hassle.
Merging the two into one project with an optional light/dark selector
seems ideal, but at least having it as a branch in the same git repo is
a step in that direction. Comparing the differences in the code base
will be much easier and bugfixes, refinements and documentation will be
much easier to port both directions.
port readme file from master branch with minor modifications
cleanup readme formatting and remove personal references
added in alerques modified powerarrow-dark