ttk is used for some, but not all, of the elements. For Buttons, you need to indicate if you want normal buttons or ttk buttons. On the Mac, ttk buttons are chosen for you because the Tk buttons have been broken on the Mac for years. So, when making a Button and you want a ttk button, set the parameter use_ttk_buttons = True.

You'll also want to change the "theme" to be the system default theme. This will turn off all colors for the window and everything in it.

This code will get you the look of window I think you're after.

import PySimpleGUI as sg

sg.theme('SystemDefaultForReal')
ttk_style = 'vista'
layout = [[sg.T(ttk_style)],
          [sg.Button('Button 1', use_ttk_buttons=True), sg.Button('Button 2', use_ttk_buttons=True)]  ]

window = sg.Window('Window Title', layout, ttk_theme=ttk_style).read(close=True)

Setting the ttk_style variable to a few of the valid values I was able to get these button styles.

https://user-images.githubusercontent.com/46163555/100234141-a500eb00-2ef8-11eb-99a5-161bdfce6de7.png

Try option image_filename or image_data of sg.Button, and set correct button color from theme setting, also set border width to 0.

Here's example how it work, remember to use your image file in following code.

import PySimpleGUI as sg

font = ('Helvetica', 12, 'bold italic')
sg.theme('Dark')
sg.set_options(font=font)
colors = (sg.theme_background_color(), sg.theme_background_color())

layout = [
    [sg.Button('Blue\nCircle', button_color=colors, image_filename='d:/circle_blue.png', border_width=0)]
]
window = sg.Window('Image Button', layout, finalize=True)

while True:
    event, values = window.read()
    if event == sg.WINDOW_CLOSED:
        break
window.close()

There is no "linking" of buttons to functions, nor callback functions.

To do what you're looking for, calling copy when you get a "Copy Button"event back from a Read.

I urge you to read through the docs to get an understanding how these calls to Button, etc, work. http://www.PySimpleGUI.org

Here's what I think you are looking for your code to do:

import PySimpleGUI as sg
import shutil, errno
src = ""
dest = ""
def copy(src, dest):
    try:
        shutil.copytree(src, dest)
    except OSError as e:
        # If the error was caused because the source wasn't a directory
        if e.errno == errno.ENOTDIR:
            shutil.copy(src, dest)
        else:
            print('Directory not copied. Error: %s' % e)

#Me testing out commands in PSG
layout = [[ sg.Text("Select path from source to destination")],
[sg.Text("Source Folder", size=(15,1)), sg.InputText(src),
sg.FolderBrowse()],
[sg.Text("Destination Folder", size=(15,1)),
sg.InputText(dest), sg.FolderBrowse()],
[sg.Button("Transfer", button_color=("white", "blue"), size=
(6, 1)),sg.Button("Copy", button_color=("white",
"green"), size=(6, 1)),sg.Exit(button_color=("white", "red"),
size=(6, 1))]]

window = sg.Window("Mass File Transfer").Layout(layout)

while True:
    event, values = window.Read()
    print(event, values)
    if event in (None, 'Exit'):
        break

    if event == 'Copy':
        copy(values[0], values[1])

Try this.

Create a button When button pressed call popup_get_file Take results from popup and fill in the Input element

import PySimpleGUI as sg

def main():
    layout = [  [sg.Text('My Window')],
                [sg.Input(key='-IN-'), sg.Button('MyBrowse', key='-BROWSE-')],
                [sg.Button('Go'), sg.Button('Exit')]  ]

    window = sg.Window('Window Title', layout)

    while True:             # Event Loop
        event, values = window.read()
        print(event, values)
        if event == sg.WIN_CLOSED or event == 'Exit':
            break
        if event == '-BROWSE-':
            file = sg.popup_get_file('', no_window=True)
            window['-IN-'].update(file)
    window.close()

if __name__ == '__main__':
    main()