Fix: Store theme in local storage after changing it in theme selector

[cappuccinocosmico]

Using this project: https://github.com/pacocoursey/next-themes

is probably best since its the only one that has actually prevented the fucking screen flicker problem, instead of the recommended solution of: https://github.com/saadeghi/theme-change which does do flicker.

This will involve hopefully a quick refactor of the theme selector