Formalize the to/from Store rules for UserStore (#2708)

- Explicitly don't save to disk if the value in the store is the same as
  the default. That way we can change the defaults between versions

- Calculate the PreferencesModel on demand

- In the future, when we turn on strict null checking, we can enforce
  that the UserStore expects the correct types too

Signed-off-by: Sebastian Malton <sebastian@malton.name>

simplify default check, remove defaults from UserStore

Signed-off-by: Sebastian Malton <sebastian@malton.name>

src/renderer/theme.store.ts

@@ -46,6 +46,7 @@ export interface ThemeItems extends Theme {
 }
 
 export class ThemeStore extends Singleton {
+  static readonly defaultTheme = "lens-dark";
   protected styles: HTMLStyleElement;
 
   // bundled themes from `themes/${themeId}.json`