--- a/toolkit/mozapps/extensions/content/extensions.js
+++ b/toolkit/mozapps/extensions/content/extensions.js
@@ -235,23 +235,17 @@ function isDisabledUnsigned(addon) {
 }
 
 function isLegacyExtension(addon) {
   let legacy = false;
   if (addon.type == "extension" && !addon.isWebExtension) {
     legacy = true;
   }
   if (addon.type == "theme") {
-    // The logic here is kind of clunky but we want to mark complete
-    // themes as legacy.  There's no explicit flag for complete
-    // themes so explicitly check for new style themes (for which
-    // isWebExtension is true) or lightweight themes (which have
-    // ids that end with @personas.mozilla.org)
-    legacy = !(addon.isWebExtension || addon.id.endsWith("@personas.mozilla.org") ||
-               addon.id == "default-theme@mozilla.org");
+    legacy = false;
   }
 
   if (legacy && (addon.hidden || addon.signedState == AddonManager.SIGNEDSTATE_PRIVILEGED)) {
     legacy = false;
   }
   // Exceptions that can slip through above: the default theme plus
   // test pilot addons until we get SIGNEDSTATE_PRIVILEGED deployed.
   if (legacy && legacyWarningExceptions.includes(addon.id)) {
@@ -2578,37 +2572,17 @@ var gDetailView = {
     this._addon = aAddon;
     gEventManager.registerAddonListener(this, aAddon.id);
     gEventManager.registerInstallListener(this);
 
     this.node.setAttribute("type", aAddon.type);
 
     let legacy = false;
     if (!aAddon.install) {
-      if (aAddon.type == "extension" && !aAddon.isWebExtension) {
-        legacy = true;
-      }
-      if (aAddon.type == "theme") {
-        // The logic here is kind of clunky but we want to mark complete
-        // themes as legacy.  There's no explicit flag for complete
-        // themes so explicitly check for new style themes (for which
-        // isWebExtension is true) or lightweight themes (which have
-        // ids that end with @personas.mozilla.org)
-        legacy = !(aAddon.isWebExtension || aAddon.id.endsWith("@personas.mozilla.org"));
-      }
-
-      if (legacy && aAddon.signedState == AddonManager.SIGNEDSTATE_PRIVILEGED) {
-        legacy = false;
-      }
-
-      // Exceptions that can slip through above: the default theme plus
-      // test pilot addons until we get SIGNEDSTATE_PRIVILEGED deployed.
-      if (legacy && legacyWarningExceptions.includes(aAddon.id)) {
-        legacy = false;
-      }
+      legacy = isLegacyExtension(aAddon);
     }
     this.node.setAttribute("legacy", legacy);
     document.getElementById("detail-legacy-warning").href = SUPPORT_URL + "webextensions";
 
     // Make sure to select the correct category
     let category = (isDisabledLegacy(aAddon) || isDisabledUnsigned(aAddon)) ?
                    "addons://legacy" : `addons://list/${aAddon.type}`;
     gCategories.select(category);

--- a/toolkit/mozapps/extensions/test/browser/browser.ini
+++ b/toolkit/mozapps/extensions/test/browser/browser.ini
@@ -65,17 +65,16 @@ skip-if = true # Bug 1449071 - Frequent 
 skip-if = os == 'linux' && !debug # Bug 1398766
 [browser_inlinesettings_browser.js]
 skip-if = (verify && debug && (os == 'mac'))
 [browser_installssl.js]
 skip-if = verify
 [browser_langpack_signing.js]
 [browser_legacy.js]
 [browser_legacy_pre57.js]
-[browser_legacy_themes.js]
 [browser_list.js]
 [browser_manualupdates.js]
 [browser_pluginprefs.js]
 [browser_pluginprefs_is_not_disabled.js]
 [browser_plugin_enabled_state_locked.js]
 [browser_recentupdates.js]
 [browser_sorting.js]
 [browser_sorting_plugins.js]

--- a/toolkit/mozapps/extensions/test/browser/browser_legacy_pre57.js
+++ b/toolkit/mozapps/extensions/test/browser/browser_legacy_pre57.js
@@ -1,28 +1,21 @@
 
 add_task(async function() {
   const INFO_URL = Services.urlFormatter.formatURLPref("app.support.baseURL") + "webextensions";
 
   const NAMES = {
-    fullTheme: "Full Theme",
     newTheme: "New LWT",
     legacy: "Legacy Extension",
     webextension: "WebExtension",
     dictionary: "Dictionary",
     langpack: "Language Pack",
   };
   let addons = [
     {
-      id: "full-theme@tests.mozilla.org",
-      name: NAMES.fullTheme,
-      type: "theme",
-      isWebExtension: false,
-    },
-    {
       id: "new-theme@tests.mozilla.org",
       name: NAMES.newTheme,
       type: "theme",
       isWebExtension: true,
     },
     {
       id: "legacy@tests.mozilla.org",
       name: NAMES.legacy,
@@ -82,16 +75,15 @@ add_task(async function() {
     if (isLegacy) {
       is_element_visible(badge, `Legacy badge is visible for ${name}`);
       is(badge.href, INFO_URL, "Legacy badge link is correct");
     } else {
       is_element_hidden(badge, `Legacy badge is hidden for ${name}`);
     }
   }
 
-  await check("theme", NAMES.fullTheme, true);
   await check("theme", NAMES.newTheme, false);
   await check("extension", NAMES.legacy, true);
   await check("extension", NAMES.webextension, false);
   await check("dictionary", NAMES.dictionary, false);
 
   await close_manager(mgrWin);
 });

deleted file mode 100644
--- a/toolkit/mozapps/extensions/test/browser/browser_legacy_themes.js
+++ /dev/null
@@ -1,77 +0,0 @@
-
-add_task(async function() {
-  // The mochitest framework installs a bunch of legacy extensions.
-  // Fortunately, the extensions.legacy.exceptions preference exists to
-  // avoid treating some extensions as legacy for the purposes of the UI.
-  const IGNORE = [
-    "special-powers@mozilla.org",
-    "mochikit@mozilla.org",
-  ];
-
-  let exceptions = Services.prefs.getCharPref("extensions.legacy.exceptions");
-  exceptions = [ exceptions, ...IGNORE ].join(",");
-
-  await SpecialPowers.pushPrefEnv({
-    set: [
-      ["extensions.legacy.enabled", false],
-      ["extensions.legacy.exceptions", exceptions],
-    ],
-  });
-
-  const ID = "theme@tests.mozilla.org";
-
-  let provider = new MockProvider();
-  provider.createAddons([{
-    id: ID,
-    name: "Complete Theme",
-    type: "theme",
-    appDisabled: true,
-  }]);
-
-  // Open about:addons and go to the themes list
-  let mgrWin = await open_manager(null);
-  let catUtils = new CategoryUtilities(mgrWin);
-  await catUtils.openType("theme");
-
-  // Our complete theme should not be displayed
-  let list = mgrWin.document.getElementById("addon-list");
-  let item = list.children.find(item => item.mAddon.id == ID);
-  is(item, undefined, `Theme ${ID} should not be in the list of active themes`);
-
-  // The warning banner and the legacy category should both be visible
-  let banner = mgrWin.document.getElementById("legacy-extensions-notice");
-  is_element_visible(banner, "Warning about legacy themes should be visible");
-  is(mgrWin.gLegacyView._categoryItem.disabled, false, "Legacy category should be visible ");
-
-  // Follow the link to the legacy extensions page
-  let legacyLink = mgrWin.document.getElementById("legacy-extensions-learnmore-link");
-  is_element_visible(legacyLink, "Link to legacy extensions is visible");
-
-  let loadPromise = new Promise(resolve => wait_for_view_load(mgrWin, resolve, true));
-  legacyLink.click();
-  await loadPromise;
-
-  is(mgrWin.gViewController.currentViewId, "addons://legacy/",
-     "Legacy extensions link leads to the correct view");
-
-  list = mgrWin.document.getElementById("legacy-list");
-  is(list.children.length, 1, "Should have 1 item in the legacy list");
-  item = list.children[0];
-  is(item.mAddon.id, ID, "Complete theme should be in the list");
-
-  // Click the find a replacement button
-  let button = document.getAnonymousElementByAttribute(item, "anonid", "replacement-btn");
-  is_element_visible(button, "Find a replacement butotn is visible");
-
-  // In automation, app.support.baseURL points to a page on localhost.
-  // The actual page is 404 in the test but that doesn't matter here,
-  // just the fact that we load the right URL.
-  let url = Services.prefs.getStringPref("app.support.baseURL") + "complete-themes";
-  let tabPromise = BrowserTestUtils.waitForNewTab(gBrowser, url);
-  button.click();
-  let tab = await tabPromise;
-  ok(true, "Find a replacement button opened SUMO page");
-  BrowserTestUtils.removeTab(tab);
-
-  await close_manager(mgrWin);
-});