[4.0] Remove alert container background

[Quy]

Replaces PR #26855 for Issue #26817 .

Summary of Changes

Remove alert container background.

Does not fix issue #25850.

Testing Instructions

Go to \administrator\templates\atum\scss\vendor\joomla-custom-elements\joomla-alert.scss
Change --atum-bg-light to --atum-bg-dark to see the alert container background
Run npm i
Under Joomla Patch Tester, install a patch to see multiple alerts with blue background between alerts.
Apply PR
Run npm i

Expected result

Transparent background.

Actual result

[brianteeman]

I have tested this item ✅ successfully on 40c43f7

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[jwaisner]

I have tested this item ✅ successfully on 40c43f7

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/27394.}

[alikon]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/27394.}

[HLeithner]

Thanks