Inconsistent highlighting of default button under tkinter?

The following Python module is designed to be used from any tkinter-based program, to pop up a dialog in front of the parent window. Of particular interest is askquestion, a substitute for the built-in messagebox version which doesn't always appear on top when it needs to. Note the Yes button, which has default="active". When run on Mac, this produces a nice, Mac like blue color to the button. When run on Ubuntu, there is absolutely no highlight to tell the user that it is the default button. Mac version:
Linux version:

Is this right? Is this a bug? Is there some parameter I might change somewhere to improve the situation? And for that matter, what is the Linux-like way to indicate a default button?

import tkinter as tk
from tkinter import ttk
from tkinter.font import Font
# This module exists to solve a problem that messagebox popups
# invoked over a child window tend to pop up over the top-level window
# instead under Linux.
messageFont = None
def pop(parent, width, height, title=""):
'''Returns an open window centered over parent waiting for you to add content.
Use child.destroy() to get rid of it when you are finished with it.'''
global messageFont
if not messageFont:
messageFont = Font(root=parent, size=16)
corner = (parent.winfo_x(), parent.winfo_y())
size = (parent.winfo_width(), parent.winfo_height())
popSize = (width, height)
popCorner = sub(add(corner, div(size, 2)), div(popSize, 2))
child= tk.Toplevel(parent)
child.attributes("-topmost", True)
geometry = f"{width}x{height}+{int(popCorner[0])}+{int(popCorner[1])}"
print(popCorner,geometry)
child.geometry(f"{width}x{height}+{int(popCorner[0])}+{int(popCorner[1])}")
child.title(title)
return child
def askquestion(parent, question, action, title=""):
def onPress(yesNo):
child.destroy()
action(yesNo)
child = pop(parent, 500, 250, title=title)
frame = ttk.Frame(child)
frame.pack(fill=tk.BOTH, expand="y")
ttk.Label(frame, text=question, font=messageFont).pack(pady=(80,0))
buttons = ttk.Frame(frame)
buttons.pack()
ttk.Button(buttons, text="No", command=lambda: onPress(False)).pack(side=tk.LEFT)
ttk.Button(buttons, text="Yes", command=lambda: onPress(True),
default="active").pack(side=tk.LEFT)
child.bind("<Escape>", lambda _event: onPress(False))
child.bind("<Return>", lambda _event: onPress(True))
# basic vector arithmetic
def add(v1, v2):
return (v1[0] + v2[0], v1[1] + v2[1])
def sub(v1, v2):
return (v1[0] - v2[0], v1[1] - v2[1])
def mul(v1, c):
return (v1[0] * c, v1[1] * c)
def div(v1, c):
return (v1[0] / c, v1[1] / c)
if __name__ == "__main__":
root = tk.Tk()
root.geometry("640x480")
root.update_idletasks()
askquestion(root, "Are you awake?", lambda yesNo: print(yesNo))
root.mainloop()

default is supported by tkinter Buttons, but not by ttk.Button. The fact that it works in MacOS is an accident.

You can try going back to tkinter.Button, or you can styling your button in a different way.

https://anzeljg.github.io/rin2/book2/2405/docs/tkinter/ttk-Button.html