Haven't checked, but wrapping these: https://searchfox.org/mozilla-central/rev/be413c29deeb86be6cdac22445e0d0b035cb9e04/toolkit/components/extensions/test/browser/browser_ext_themes_findbar.js#109-110 in a `if (!gProton) {` check should be enough, the border-color prop shouldn't be applied with Proton on the prev/next buttons.
Bug 1703411 Comment 2 Edit History
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Haven't checked, but wrapping these: https://searchfox.org/mozilla-central/rev/be413c29deeb86be6cdac22445e0d0b035cb9e04/toolkit/components/extensions/test/browser/browser_ext_themes_findbar.js#88-91,109-110 in a `if (!gProton) {` check should be enough, the border-color prop shouldn't be applied with Proton on the prev/next buttons.