The UI Builder is based on different components including MUI and custom HTML components.
The designer has a common interface customizing components/blocks, therefore there are not all properties from MUI and/or there are other properties.
Adding ThemeExtensions (Styles) is the way to go to override the basic appearance
https://backendless.com/docs/uibuilder/ui_extensions.html
Normally, each sub-component (DOM element) has its own CSS class which starts with bl- we use them to apply the default theme and you can use them to apply your own styles
Regards,
Vlad