[5.0] Fix color of plus symbol in dark mode

[wilsonge]

Pull Request for Issue raised by @Quy in #41409 .

Summary of Changes

Fixes the color of the plus symbol so it's visible

Testing Instructions

Log into atum and check the color of the plus symbol in the dashboard.

Actual result BEFORE applying this Pull Request

Note the plus symbol pointed to by the red arrow is effectively invisible (ignore the other arrow at the success box for this PR)

Expected result AFTER applying this Pull Request

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[Quy]

I have tested this item ✅ successfully on 7f9677e

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