Use rstudio.sessionInit hook instead of later

[gadenbuie]

# theme options based on hour of day -------------------------------------------
# based on https://github.com/rstudio/rstudio/issues/1579
setHook("rstudio.sessionInit", function() {
  # ...
})

Thanks to @mine-cetinkaya-rundel: https://gist.github.com/mine-cetinkaya-rundel/465335e8ff4d0e8beb38b296a6ac712d