If you want to implement this feature in your project: (maybe first create a copy of your project) Remix OR: You have the grab the two sprites "notify box" and "notify letters" in your backpack. Then put the two sprites in another project. If you want to show a notification, you only have to change the varibale "text to notify" to a text, you want to show. Then broadcast "notify this". To change the box color, use the variable "black(1), white(2) notify box". This add on works in Scratch 2 and Scratch 3 with upper and lower case. Notes: I'm not responsible for false implementing or errors. Don't spam the broadcast "notify this", wait until the message was sent. Some letters like: ÄÖÜ and special characters aren't supported.
Font: ZCOOL QingKe HuangYou Rectangle made with Lunacy. // Animated-Thumbnail-Feature by @World_Languages