gnulinux: add support for system theme

Use 'color-scheme' from gsettings if available to check if dark theme
is preferred.  Otherwise, fall back on theme name from 'gtk-theme'.

The default is to follow the system theme (if supported), and
'EnableDarkTheme' would be unused.  'AppTheme' can currently have
one of three values: 'System', 'Dark', 'Light'.

Change-Id: I8008c8fe0f4750c97e71838fff0031f80b685f5e
GitLab: #723