IDLE line numbering should be light gray

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2020-06-03.17:10:44.010>
labels = ['expert-IDLE', 'type-bug', '3.9', '3.10']
title = 'IDLE line numbering should be light gray'
updated_at = <Date 2020-06-04.08:08:25.101>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2020-06-04.08:08:25.101>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2020-06-03.17:10:44.010>
creator = 'rhettinger'
dependencies = []
files = []
hgrepos = []
issue_num = 40856
keywords = []
message_count = 4.0
messages = ['370681', '370686', '370688', '370699']
nosy_count = 4.0
nosy_names = ['rhettinger', 'terry.reedy', 'taleinat', 'cheryl.sabella']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40856'
versions = ['Python 3.9', 'Python 3.10']