feat: implement dark theme with manual/auto modes

• set an automatic class in the body, instead of a static dark theme class definition, because it didn't match the prior definition of a relevant theme for the user
• added theme initialization logic to the App.svelte root component, which blocks the first render of the application until the theme initialization is complete. This is fast, prevents re-renders on cold boot, and the user gets an immediately matching theme on the first render
• implementation of automatic theme detection mode, done by listening to CSS Media Query in JS
• working with user's themeization data via LocalStorage

Closes #36