Pull Request resolves #43894 .

• I read the Generative AI policy and my contribution is either not created with the help of AI or is compatible with the policy and GNU/GPL 2 or later.

Summary of Changes

Move the Atum accessibility classes from the element to the element.

The monochrome and a11y_contrast classes apply CSS filters. Applying a filter to <body> creates a containing block for position: fixed descendants, causing fixed-position elements such as the Debug System icon and window to be positioned relative to the document instead of the viewport.

The accessibility classes are now applied to <html>, which prevents the filter from interfering with position: fixed elements.

The existing accessibility functionality and visual behaviour are otherwise unchanged.

Testing Instructions

Enable debug in global config
Observe that the debug plugin is fixed to the bottom of the viewport
In your admin user profile.
Go to the Accessibility Settings t5ab and Enable Monochrome.
Go to the admin home dashboard

As this PR includes scss changes you will need to either use a prebuiilt package or npm i to rebuild the css

Actual result BEFORE applying this Pull Request

The debug plugin has moved to the bottom of the html not fixed to the bottom of viewport

Expected result AFTER applying this Pull Request

The debug plugin is fixed to the bottom of viewport

Link to documentations

Please select:

• Documentation link for guide.joomla.org:

• No documentation changes for guide.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed