Sample for simple modifier

ContentView.swift

import SwiftUI

struct ContentView: View {
    var body: some View {
        VStack {
            globeLabel
            .padding()
            .mainCell() // Applying quite simple custom modifier
        }
    }

    private var globeLabel: some View {
        Label("Hello, world!", systemImage: "globe")
    }
}

MainCellModifier.swift

import SwiftUI

extension View {
    func mainCell() -> some View {
        background(
            Color.cyan.opacity(0.1)
        )
        .cornerRadius(20)
    }
}

// or implement like this ...
extension View {
    func mainCell2() -> some View {
        modifier(MainCellModifier())
    }
}

private struct MainCellModifier: ViewModifier {
    func body(content: Content) -> some View {
        content.background(
            Color.cyan.opacity(0.1)
        )
        .cornerRadius(20)
    }
}