Needs ability to switch from light to dark theme.

[burdittw]

Would really be great if this would allow the user to select a theme when using an app built with these components like the actual AdminLTE. I was able to get the dark them enabled by extracting all the .dark-mode CSS out of the AdminLTE.css file. This still left some issues like the Navbar not being able to select a dark mode that I can figure out. The page title still wanted to be dark because of an !important flag in this component in the .text-dark CSS.