I test it on macOS ,It should works on Windows too.

• Replace the 'app.asar' file

You can found the app.asar file in the /Applications/Unity Hub.app/Contents/Resources

• install asar

npm i -g asar

• unpack app.asar file

You can found the app.asar file in the /Applications/Unity Hub.app/Contents/Resources

asar extract app.asar app

• Change the style

Add Custom.css to /build/renderer/index.html

• Pack you file

asar pack app app.asar

• Replace the app.asar file

MIT