UI problem in nested entries

Description

When working on medium or small screen, the entries from a structured list in the CP are difficult to manipulate.
The problem occurs in the latest version of every majors browsers (Chrome, Safari, Firefox…)

Large screens

When displaying in large screen, the nested entries are very easy to figure out and to manipulate. Nested entry level 1, 2 and 3 are clearly identifiable here.

Medium screens (like MacBook)

In a medium size screen, the "open" and "closed" marks are very hard to distinguish and there is no way to know the indent level of an entry.
Clients never find by themselves that the "open" and "close" marks are interactive.

Small screens

Nested entries can't be opened or closed in a small screen.

Additional info

• Craft version: Craft Pro 3.3.7
• PHP version: 7.2.16
• Database driver & version: MySQL 5.7.27
• Plugins & versions:
  Activity Log 1.0.1
  Control Panel CSS 2.2.0
  Default Dashboard 1.0.5
  Embedded Assets 2.0.12
  Imager v2.3.0
  inflexions-backup 1.3.5
  Migration Manager 3.0.20.1
  Redactor 2.4.0
  Redirect plugin for Craft 3 1.0.23
  SEOmatic 3.2.27

Start by reproducing structured-list entries in the Control Panel on medium and small screens with Chrome, Safari, and Firefox, using the supplied screenshots as the visual reference. Done means nested levels are distinguishable and the open/closed controls remain discoverable and usable at those screen sizes.