fix: themes broken when localStorage has invalid theme id stored

[Pistonight]

Closes #2461

Check if stored theme is valid in get_theme() and defaults to default theme if not valid.

Validation steps:

• cargo run -- build guide and serve the book
• In browser devtools, run localStorage.setItem("mdbook-theme", "foo") to set an invalid theme foo
• Refresh the page, themes should work properly:
  • the default theme should load
  • the theme popup functions properly

Manually tested in Google Chrome, MS Edge and Firefox (I don't have access to safari :(

Other:

• localStorage won't be updated (i.e. keeps the invalid value) until user manually changes the theme

[ehuss]

Thanks!