[PP-1] Labels for accent color choices on settings form are hidden from accessible technology

Problem/Motivation

First draft, to be fleshed out...

Labels for accent color choices on the theme settings form are totally hidden with display: none.

This occurs both:

• On the admin page settings form.
• On the user settings form when the "Users can override Default Admin settings" option is enabled (therefore, regular users can encounter this).

WCAG success criterion

SC 4.1.2 Name, Role, Value (Level A)

Deque impact rating: Critical

Accessibility Insights report

This is from the user settings form case, and for only one of the color options. There are twelve errors total.

Title: WCAG 4.1.2: Ensure every form element has a label (#edit-preset-accent-color-blue)
Tags: Accessibility, WCAG 4.1.2, label

Issue: Ensure every form element has a label (label - https://accessibilityinsights.io/info-examples/web/label)

Target application: admin | Drush Site-Install - https://drupal-core-dev.ddev.site/user/1/edit

Element path: #edit-preset-accent-color-blue

Snippet: <input data-drupal-selector="edit-preset-accent-color-blue" data-gin-accent="blue" style="--accent-base: #015efe;" type="radio" id="edit-preset-accent-color-blue" name="preset_accent_color" value="blue" checked="checked" class="form-radio form-boolean form-boolean--type-radio">

How to fix: 
Fix all of the following:
  Form element has explicit <label> that is hidden

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This accessibility issue was found using Accessibility Insights for Web 2.46.0 (axe-core 4.10.2), a tool that helps find and fix accessibility issues. Get more information & download this tool at http://aka.ms/AccessibilityInsights.

Steps to reproduce

• Install Drupal with the Standard profile.
• Log in as admin.
• Install the Default Admin theme and set it as the administration theme.
• Go to /admin/appearance/settings/default_admin.
• Run a test with the Axe-core or Accessibility Insights extensions.

Proposed resolution

The labels are currently hidden by the theme's custom CSS.

I propose removing this CSS from the theme and hiding them instead with the visually-hidden class.

Hiding them visually can probably accomplished by adding '#title_display' => 'invisible' for the field(s) in the form definition (Settings::getSettingsForm()).

Because this requires CSS changes, I'm postponing based on #3582351: [Meta] Clean up CSS within Admin theme.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet