Themes for SwiftUI

Themes.swift

struct Colors {
    let primary: Color
}

struct Fonts {
    let body: Font
}

class Theme: ObservableObject {
    let color: Colors
    let font: Fonts
    init(color: Colors, font: Fonts) {
        self.color = color
        self.font = font
    }
}

extension Theme {
    static let standard = Theme(
        color: Colors(primary: .blue),
        font: Fonts(body: .body)
    )
    static let alternate = Theme(
        color: Colors(primary: .red),
        font: Fonts(body: .body)
    )
}

class ThemeManager: ObservableObject {
    @Published var current: Theme = .standard
}

struct ContentView: View {
    @StateObject var themes = ThemeManager()
    var body: some View {
        NavigationView {
            AccountListLoadingView()
                // allow theme management
                .environmentObject(themes)
                // allow easy access to current theme
                .environmentObject(themes.current)
        }
    }
}

struct PrimaryLabelStyle: ViewModifier {
    @EnvironmentObject var theme: Theme
    func body(content: Content) -> some View {
        content
            .font(theme.font.body)
            .foregroundColor(theme.color.primary)
    }
}

struct AccountListCellView: View {
    let account: Account
    var body: some View {
        Text(account.id)
            .modifier(PrimaryLabelStyle())
    }
}

struct AccountListCellView_Previews: PreviewProvider {
    static var previews: some View {
        Group {
            AccountListCellView(account: Account())
                .environmentObject(Theme.standard)
            AccountListCellView(account: Account())
                .environmentObject(Theme.alternate)
        }
    }
}

struct SomeView: View {
    @EnvironmentObject var themes: ThemeManager
    var body: some View {
        Button {
            themes.current = .alternate
        } label: {
            Text("Switch Theme")
        }
    }
}