Adding Auto appearance mode to Alto

[JustMaris]

I was personally interested in adding Automatic mode to Alto, because I enjoy the minimalistic look of the theme, but I didn't want to be limited only to Light and Dark mode, as I myself have started enjoying automatic appearance. Due to this, I did work to implement this logic. Logic in the header is for the initial load. In the footer for continuous adjustment, if the change happens during browsing the page.

I'm happy to respond to any concerns or to make adjustments.