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If you use your web-browser’s Web Developer Tools to Inspect the HTML elements that make up the left ‘sidebar’ you will see they look something like…
The 2nd CSS example you supplied targets the <tw-sidebar> element, which will effect all of the ‘sidebar’ content.
The 1st example you supplied, that uses a .redo CSS class, used to be how you would effect the Redo link, however the <tw-icon> elements that represent the Undo & Redo links were changed in a recent version of Harlowe. You now need a CSS selector that targets the title property of the element, for the Redo link that would look something like…