Fix background color on vertical overflow

[scottalguire]

resolves #115

Changes

• using a useEffect, toggles application of the dark className on the html element whenever darkMode state is updated, (be it onload, or state change)
• added conditional tailwind classes to the body element bg-background dark:bg-background-dark
• removed deprecated background-color key in the index css

Notes

• This methodology for accessing the html tag is widely supported

Sample