Add Varbase Installer Theme for use during installation on Drupal 11.2.0+

Problem/Motivation

As of Drupal ~11.2.0, the install process supports using dedicated install themes.
To restore Varbase's original branded installation experience while leveraging Gin as the base,
a new minimal install theme is needed. This ensures the installation flow remains familiar,
styled, and modern without impacting the post-install admin UX.

Hooks to while installing are limited after ~11.2.0
Adding style or script is no longer an option.

Before the fix:

Proposed resolution

Following with the new logic from the Drupal CMS, after the change in Drupal ~11.2.0

Introduced a Varbase Installer Theme that extends the Gin admin theme and is used
only during installation, for Drupal versions 11.2.0 and later.

Overview:

This minimal theme:

• Inherits all functionality from Gin
• Adds original Varbase install-page.css and install-scripts.js styling
• Is scoped to the install phase only

Features:

• Minimal Override: Keeps Gin fully intact as the base theme
• Installation-Only: Automatically disabled after installation
• Original Varbase Branding: Includes familiar install-page styling and behavior

File Structure:

varbase_installer_theme/
├── css/install-page.css
├── js/install-scripts.js
├── varbase_installer_theme.info.yml
├── varbase_installer_theme.libraries.yml
└── README.md

How It Works:

1. Extends Gin to maintain consistency with the admin experience.
2. Adds a custom library to Gin’s install-page to inject Varbase install styling.
3. Used only during installation for Drupal 11.2.0+ via install key in varbase.info.yml.

This update ensures a clean, modern, and branded install process for Varbase while leveraging core enhancements in Drupal 11.2+.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ✅ UX/UI designer responsibilities
• ✅ Accessibility and Readability
• ✅ Code review from 1 Varbase core team member
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes and Update Helper on new feature change/addition
• ✅ Release varbase-10.1.0-alpha2

Varbase update type

• ✅ No Update
• ➖ Optional Update
• ➖ Forced Update
• ➖ Forced Update if Unchanged

User interface changes

After the fix:

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Issue #3539556: Added Varbase Installer Theme for use during installation on Drupal 11.2.0+