Hello,
There are a few changes you can make to achieve your desired effect. Please note that you will have to host and modify your own instance of Unified-RED to make some of these changes. Instructions for this can be found in the "Developers" section of the README on the public repository for Unified-RED (linked here).
To modify logos, replace the following files with your desired versions:
/.node-red/node_modules/unified-red/dist/assets/images/favicon.ico/.node-red/node_modules/unified-red/dist/assets/images/logo.png
/.node-red/node_modules/unified-red/dist/assets/images/pages/unified-logo.png
/unified-red/src/assets/images/favicon.ico
/unified-red/src/assets/images/logo.png
/unified-red/src/assets/images/pages/unified-logo.png
To modify text in the login screen, make changes to the following files:
/unified-red/src/app/authentication/reset-password/reset-password-component.html
/unified-red/src/app/authentication/forgot-password/forgot-password-component.html
/unified-red/src/app/authentication/login/login-component.html
When modifying these files, I found that replacing images with much larger ones resulted in parts of that image being hid behind other elements. Please be sure to match the source resolution when overwriting an image.
I hope you find this response helpful. Please reply with any further questions.