Bug fixes and minor improvements.

A couple fixes and improvement. Some of these PRs were made a while ago, but I forgot to release them.

Bug fixes:

• Fix dark theme / optional search bar #21 (thanks @matteozappia and @JohJakob!)
• Fix compile error with non-optional for Swift 5 (@javiergarmon)
• Fix macOS version check #3 (@orchetect)

New features:

• Icon support for SettingButton #12 (@Saik0s, @joekw)
• Conditional NavigationStack support #14 (@mpdifran)
• Menu picker support #10 (@n3d1117)
• Theme entire setting stack #7

Thanks for the PRs everyone!