#435 web dark mode (#455)

* add ThemeProvider to App.web.tsx

* make FlatNavigator use themed color

* fix extra padding on top in web

* add observer to App.web.tsx to make it react to theme changes

* fix TS for useColorSchemeStyle

* add dark mode toggle button to web LeftNav

* fix index.web.tsx border colors for web

* Move the darkmode desktop web toggle to the right nav column

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Co-authored-by: Paul Frazee <pfrazee@gmail.com>

src/lib/hooks/useColorSchemeStyle.ts

@@ -1,6 +1,6 @@
 import {useTheme} from 'lib/ThemeContext'
 
-export function useColorSchemeStyle(lightStyle: any, darkStyle: any) {
+export function useColorSchemeStyle<T>(lightStyle: T, darkStyle: T) {
   const colorScheme = useTheme().colorScheme
   return colorScheme === 'dark' ? darkStyle : lightStyle
 }