Customizing ToolBar of SplitView

Currently you can't customize the appearance sidebarToggle. Please file an enhancement request – If you do so, please share your report ID here for folks to track.

You are allowed to remove the built in sidebarToggle using .toolbar(removing: .sidebarToggle) and create a custom toolbar item using ToolbarItem(placement: .navigation).

However It won't completely appear as the built in sidebarToggle and it won't have the same behaviour.

Here's an example:

struct ContentView: View {
    @State private var columnVisibility: NavigationSplitViewVisibility = .all

    var body: some View {
        NavigationSplitView(columnVisibility: $columnVisibility) {
            List {
                Text("Hello world")
            }
            .toolbar(removing: .sidebarToggle)
        } detail: {
            Text("Detail")
        }
        .toolbar {
            ToolbarItem(placement: .navigation) {
                Button {
                    withAnimation {
                        toggleSidebar()
                    }
                } label: {
                    Image(systemName: "sidebar.left")
                }
            }
        }
    }
    
    private func toggleSidebar() {
        if columnVisibility == .all {
            columnVisibility = .detailOnly
        } else {
            columnVisibility = .all
        }
    }
}