When advancements use dark items for their icons, the icon has low contrast against the toast background.
This can be seen with the "The Next Generation" advancement toast:
[media]
A solution could be to ship two toast background textures (dark and light) and let advancement displays optionally specify a "use_light_toast_background" boolean.
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Can confirm in 25w02a, relates to MC-275339.

Thank you for your report!
After consideration, the issue is being closed as Won't Fix.
Please note that this is not the same as Working as Intended, as this bug report correctly describes behavior in the game that might not be the intended or desirable behavior, but it will not be fixed right now. Sometimes, this is because the issue reported is minor and/or impossible to change without large architectural changes to the code base.
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Kluckings, please refer to MC-275339, bad visibility/contrast making it hard to see items in UI is a valid issue to report.