Is there a way to have a link in the users preferences to allow the use to choose from a Light or dark theme? and its saved for that user?
No, but it would be a good beginner plugin. Or if you write your own theme, the ThemeHooks file can be used for that. A theme hooks file is nothing more than a theme specific plugin.
You have chosen great patience when debugging the htaccess error, so if you want some support writing such a plugin, just drop a line. I will happily guide you.
If not, just say so. Maybe in a few days I'll have the time to throw something like that together. I have written a theme switcher in the past, but the purpose was kind of different and I think somebody has written a chose option for the bootstrap theme subthemes. So you might find something helpful in the plugins
I too was going to mention that I never want to owe @MrCaspan any money, or I would be troubleshot very precisely to the fourth generation. I salute your endurance and excellent patience.
As for theme switcher I would go client side ...
I'm not sure if I should be insulted or not :) I can be a hammer looking for a nail and believe me when I know something should work I rip it apart to figure out why its not. Its why i do IT and i can normally figure out most stuff!
There, sir, was a compliment of most high esteem and rare calibre. Let's just say I read your whole thread even though it did not concern me.
... when I know something should work I rip it apart to figure out why its not
lol ... my very same motto indeed, only with a slight variation ...
when I know something should work, I beat the dying daylight out of it until it starts to work; then I shudder at all those around me who hitherthen made it seem so pretentiously complicated or impossible. In other words, where there should be a way, Will I Am ...