fix: address QA issues

ts/components/conversation/right-panel/overlay/disappearing-messages/OverlayDisappearingMessages.tsx

@@ -28,7 +28,7 @@ const ButtonSpacer = styled.div`
 `;
 
 const StyledButtonContainer = styled.div`
-  background: linear-gradient(0deg, black, transparent);
+  background: linear-gradient(0deg, var(--background-primary-color), transparent);
   position: absolute;
   width: 100%;
   bottom: 0px;

ts/models/conversation.ts

@@ -888,7 +888,9 @@ export class ConversationModel extends Backbone.Model<ConversationAttributes> {
         !PubKey.isClosedGroupV3(this.id) &&
         !fromConfigMessage &&
         expirationMode !== this.get('expirationMode') &&
-        expireTimer !== this.get('expireTimer'));
+        expireTimer !== this.get('expireTimer') &&
+        expirationMode !== 'off' &&
+        expireTimer !== 0);
 
     // When we add a disappearing messages notification to the conversation, we want it
     // to be above the message that initiated that change, hence the subtraction.