Add automatic dark theme for website.

[mike-jumper]

This change adds a dark mode to the Guacamole website that will be chosen by the browser automatically whenever the user's system is set to dark mode.

This is accomplished by:

• Generating the same CSS twice - once for dark mode and again for light mode.
• Defining a $mode variable and $globals.lightdark() function that selects between two choices depending on whether we are generating CSS for light mode or dark mode.
• Pulling the colors out into variables which are conditional leveraging that function.

Light	Dark