fix: fallback to user prefered mode instead of assuming dark mode for first-time theme load

[ZibanPirate]

The bug:

clear the cookie and load the website on a system that has light theme preferred

Expected:
you should get light mode

Actual:
you get dark mode

Fix:

when loading the theme from cookie, if no theme is found, try detecting user-prefered theme, and then fallback to dark mode

NOTE

also changed the code a bit, to allow for non-binary options, like having a thrid auto-detect option, or other theme names

• Bug fix
• New feature
• Other