Subclass of NSButton

FlatButton.swift

import AppKit
public class FlatButton: NSButton {
 
    public var buttonColor: NSColor = NSColor(calibratedRed: 0.201, green: 0.404, blue: 0.192, alpha: 1)
    public var onClickColor: NSColor = NSColor(calibratedRed: 0.304, green: 0.601, blue: 0.294, alpha: 1)
    public var textColor: NSColor = NSColor.white
    
    public override func draw(_ dirtyRect: NSRect) {
        super.draw(dirtyRect)
     
        let rectanglePath = NSBezierPath(rect: NSRect(x: 0, y: 0, width: self.frame.width, height: self.frame.height))
     
        var fillColor: NSColor
        var strokeColor: NSColor
        
        rectanglePath.fill()
        
        if self.isHighlighted {
            strokeColor = self.buttonColor
            fillColor = self.onClickColor
        } else {
            strokeColor = self.onClickColor
            fillColor = self.buttonColor
        }
     
        strokeColor.setStroke()
        rectanglePath.lineWidth = 5
        rectanglePath.stroke()
        fillColor.setFill()
        rectanglePath.fill()
        bezelStyle = .shadowlessSquare
     
        let textRect = NSRect(x: 0, y: 0, width: self.frame.width, height: self.frame.height)
        let textTextContent = self.title
        let textStyle = NSMutableParagraphStyle()
        textStyle.alignment = .center
        
        let textFontAttributes : [ NSAttributedString.Key : Any ] = [
          .font: NSFont(name: "SFUIText-Medium", size: NSFont.systemFontSize)!,
          .foregroundColor: textColor,
          .paragraphStyle: textStyle
        ]

        let textTextHeight: CGFloat = textTextContent.boundingRect(with: NSSize(width: textRect.width, height: CGFloat.infinity), options: .usesLineFragmentOrigin, attributes: textFontAttributes).height
        let textTextRect: NSRect = NSRect(x: 0, y: -3 + ((textRect.height - textTextHeight) / 2), width: textRect.width, height: textTextHeight)
        NSGraphicsContext.saveGraphicsState()
        textTextContent.draw(in: textTextRect.offsetBy(dx: 0, dy: 3), withAttributes: textFontAttributes)
        NSGraphicsContext.restoreGraphicsState()
    }
}

If we can have this updated to the latest version, it would be really helpful.

I deleted the comment because I found some time to update the code anyway. I would just recommend reading the documentation provided by Apple. The HIG is the go to resource for that.
https://developer.apple.com/design/human-interface-guidelines/macos/overview/themes/

I would recommend StackOverflow for specific questions.

Even SwiftUI is in it's infancy and has a lot of bugs and frustrating constraints, the power of coding the same "language" for all devices across the Apple ecosystem is a real plus. I say "language", because the difference between iOS, tvOS, macOS and watchOS for that sake, is so far from each other that it's almost like different dialects of the same language. So for better or worse, I went all inn for SwiftUI.

It helps me a lot, thank you so much!