IDLE: Handle bad highlight tab color config

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2020-07-29.03:33:23.391>
labels = ['expert-IDLE', 'type-bug', '3.10']
title = 'IDLE: Handle bad highlight tab color config'
updated_at = <Date 2020-07-29.03:33:23.391>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2020-07-29.03:33:23.391>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2020-07-29.03:33:23.391>
creator = 'terry.reedy'
dependencies = []
files = []
hgrepos = []
issue_num = 41432
keywords = []
message_count = 1.0
messages = ['374558']
nosy_count = 1.0
nosy_names = ['terry.reedy']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41432'
versions = ['Python 3.10']