fix theme control arrow selectors #1257

css/reveal.css

css/reveal.scss

css/theme/beige.css

css/theme/black.css

css/theme/blood.css

css/theme/league.css

css/theme/moon.css

css/theme/night.css

css/theme/serif.css

css/theme/simple.css

css/theme/sky.css

css/theme/solarized.css

css/theme/template/theme.scss

css/theme/white.css