Hi @Ayakashi97 I probably forgot to include in my Guide, that the smaller the base64 image file, the easier it will be for Kibana to process the file you are modifying. I recommend a size no larger than 100px. So first, reduce the original image file to 100px, then convert it to base64 and you will see that the generated code is much smaller. Pay special attention to the portion of code you need to remove from file kibana/src/core/target/public/core.entry.js, as if is not done right, it might cause to load a blank page.
And for the command npx, just install package npm first, as npx is part of it 
I am also waiting for the official way to customize the interface, so we do not have the need of these hacky solutions 
Cheers!