Fix a bug with the night-mode toggle in the Settings panel
Convert isDark into a function

This removes the hassle of changing both this.options.lightOrDark
and this.isDark
Slight fixes to style
Only set the option if RESStorage has been loaded
.redditDark() -> .enableNightMode() and .disableNightMode()

.redditDark(true) looks like it enables night mode, when it actually
*disables* night mode
Use getElementById
Merge pull request #667 from mc10/night-mode

Fix a bug with the night-mode toggle in the Settings panel