IDLE fix colors for MacOS dark mode

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2021-01-05.12:54:54.117>
labels = ['easy', 'type-bug', '3.8', '3.9', '3.10', 'expert-IDLE']
title = 'IDLE fix colours for MacOS dark mode'
updated_at = <Date 2021-05-28.20:14:35.351>
user = 'https://github.com/E-Paine'

bugs.python.org fields:

activity = <Date 2021-05-28.20:14:35.351>
actor = 'taleinat'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2021-01-05.12:54:54.117>
creator = 'epaine'
dependencies = []
files = []
hgrepos = []
issue_num = 42831
keywords = ['easy', 'newcomer friendly']
message_count = 3.0
messages = ['384391', '384416', '384420']
nosy_count = 3.0
nosy_names = ['terry.reedy', 'taleinat', 'epaine']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42831'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']