Add default theme

[PaulWalkerUK]

Being able to set a theme is great, but I couldn't see any way to "remove" a theme to go back to the standard default. I've added a Default theme which sets the colours to the original, standard, default values. Note that this uses the base, out-of-the box colours - if someone has made changes to their Default session, this will ignore that - it doesn't attempt to do anything fancy like find out what the user's current default colours are