Fieldset grey background in Chrome 87.x [#3184261]

Problem/Motivation

In Chrome 87.0.4280.66 fieldsets nested inside details elements (and possibly other places) have a strange grey background that isn't seen in other browsers.

Steps to reproduce

Open any Drupal form using Claro theme which has fieldsets inside details elements and notice the grey background.

Proposed resolution

Specifying a non rgba border color on fieldsets seems to fix it, e.g. border-color: rgb(216, 217, 224);

Remaining tasks

Patch needed.

A user submitted a similar issue on #3183988: Fix Claro admin theme style of gray fieldset on Google Chrome 87

Screenshot of grey background issue in Chrome

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