--- a/toolkit/mozapps/extensions/internal/XPIProvider.jsm
+++ b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
@@ -260,16 +260,17 @@ const SIGNED_TYPES = new Set([
   "experiment",
   "webextension",
   "webextension-theme",
 ]);
 
 const LEGACY_TYPES = new Set([
   "apiextension",
   "extension",
+  "theme",
 ]);
 
 const ALL_EXTERNAL_TYPES = new Set([
   "dictionary",
   "extension",
   "experiment",
   "locale",
   "theme",

--- a/toolkit/mozapps/extensions/test/xpcshell/test_legacy.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_legacy.js
@@ -14,16 +14,23 @@ add_task(async function test_disable() {
       multiprocessCompatible: true,
     },
     {
       id: "apiexperiment@tests.mozilla.org",
       name: "WebExtension Experiment",
       version: "1.0",
       type: 256,
     },
+    {
+      id: "theme@tests.mozilla.org",
+      name: "Theme",
+      version: "1.0",
+      type: 4,
+      internalName: "mytheme/1.0",
+    },
   ];
 
   let nonLegacy = [
     {
       id: "webextension@tests.mozilla.org",
       manifest: {
         applications: {gecko: {id: "webextension@tests.mozilla.org"}},
       },